diff --git a/.clang-format b/.clang-format
index cf00c631a..c8b69fbbf 100644
--- a/.clang-format
+++ b/.clang-format
@@ -43,7 +43,7 @@ TabWidth: 4
 UseTab: Never
 ---
 Language: Cpp
-Standard: c++17
+Standard: c++20
 
 AccessModifierOffset: -4
 PackConstructorInitializers: Never
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 894ccd075..d84e37cde 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -174,7 +174,7 @@ if(NOT CMAKE_C_STANDARD)
     set(CMAKE_C_STANDARD_REQUIRED ON)
 endif()
 if(NOT CMAKE_CXX_STANDARD)
-    set(CMAKE_CXX_STANDARD 17)
+    set(CMAKE_CXX_STANDARD 20)
     set(CMAKE_CXX_STANDARD_REQUIRED ON)
 endif()
 
diff --git a/Transmission.xcodeproj/project.pbxproj b/Transmission.xcodeproj/project.pbxproj
index 94ce78b2f..5e69035c4 100644
--- a/Transmission.xcodeproj/project.pbxproj
+++ b/Transmission.xcodeproj/project.pbxproj
@@ -483,6 +483,7 @@
 		EDC37BCD2EE9C2AD001E2612 /* api-compat.cc in Sources */ = {isa = PBXBuildFile; fileRef = EDC37BCC2EE9C2AD001E2612 /* api-compat.cc */; };
 		EDC37BCE2EE9C2AD001E2612 /* api-compat.h in Headers */ = {isa = PBXBuildFile; fileRef = EDC37BCB2EE9C2AD001E2612 /* api-compat.h */; };
 		EDC749F92D98AE3000A12D0F /* PowerManager.mm in Sources */ = {isa = PBXBuildFile; fileRef = EDC749F82D98AE2900A12D0F /* PowerManager.mm */; };
+		EDD735D62D83087400852628 /* UniformTypeIdentifiers.framework in Frameworks */ = {isa = PBXBuildFile; fileRef = EDD735D52D83087400852628 /* UniformTypeIdentifiers.framework */; settings = {ATTRIBUTES = (Weak, ); }; };
 		F11545ACA7C4D7A464F703AB /* block-info.h in Headers */ = {isa = PBXBuildFile; fileRef = 6A044CBD8C049AFCBD4DB411 /* block-info.h */; settings = {ATTRIBUTES = (Project, ); }; };
 		F63480631E1D7274005B9E09 /* Images.xcassets in Resources */ = {isa = PBXBuildFile; fileRef = F63480621E1D7274005B9E09 /* Images.xcassets */; };
 /* End PBXBuildFile section */
@@ -1478,6 +1479,7 @@
 		EDC37BCC2EE9C2AD001E2612 /* api-compat.cc */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.cpp.cpp; path = "api-compat.cc"; sourceTree = "<group>"; };
 		EDC749F72D98ADE200A12D0F /* PowerManager.h */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.c.h; path = PowerManager.h; sourceTree = "<group>"; };
 		EDC749F82D98AE2900A12D0F /* PowerManager.mm */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.cpp.objcpp; path = PowerManager.mm; sourceTree = "<group>"; };
+		EDD735D52D83087400852628 /* UniformTypeIdentifiers.framework */ = {isa = PBXFileReference; lastKnownFileType = wrapper.framework; name = UniformTypeIdentifiers.framework; path = System/Library/Frameworks/UniformTypeIdentifiers.framework; sourceTree = SDKROOT; };
 		F63480621E1D7274005B9E09 /* Images.xcassets */ = {isa = PBXFileReference; lastKnownFileType = folder.assetcatalog; name = Images.xcassets; path = Images/Images.xcassets; sourceTree = "<group>"; };
 /* End PBXFileReference section */
 
@@ -1505,6 +1507,7 @@
 			isa = PBXFrameworksBuildPhase;
 			buildActionMask = 2147483647;
 			files = (
+				EDD735D62D83087400852628 /* UniformTypeIdentifiers.framework in Frameworks */,
 				C87369652809984200573C90 /* UserNotifications.framework in Frameworks */,
 				8D11072F0486CEB800E47090 /* Cocoa.framework in Frameworks */,
 				4D1838DD09DEC0E80047D688 /* libtransmission.a in Frameworks */,
@@ -2185,6 +2188,7 @@
 		A2F35BBA15C5A0A100EBF632 /* Frameworks */ = {
 			isa = PBXGroup;
 			children = (
+				EDD735D52D83087400852628 /* UniformTypeIdentifiers.framework */,
 				C87369642809984200573C90 /* UserNotifications.framework */,
 				55869925257074EC00F77A43 /* libcurl.tbd */,
 				C88771AB2803EE53005C7523 /* libiconv.tbd */,
@@ -4381,8 +4385,9 @@
 				LD_RUNPATH_SEARCH_PATHS = "@executable_path/../Frameworks";
 				OTHER_CPLUSPLUSFLAGS = (
 					"$(inherited)",
-					"-fmodules",
-					"-fcxx-modules",
+					"-fno-modules",
+					"-Xclang",
+					"-fno-cxx-modules",
 				);
 				PRODUCT_NAME = Transmission;
 				SYSTEM_HEADER_SEARCH_PATHS = (
@@ -4457,7 +4462,7 @@
 			buildSettings = {
 				ALWAYS_SEARCH_USER_PATHS = NO;
 				CLANG_ANALYZER_LOCALIZABILITY_NONLOCALIZED = YES;
-				CLANG_CXX_LANGUAGE_STANDARD = "c++17";
+				CLANG_CXX_LANGUAGE_STANDARD = "c++20";
 				CLANG_ENABLE_OBJC_ARC = YES;
 				CLANG_ENABLE_OBJC_WEAK = YES;
 				CLANG_WARN_BLOCK_CAPTURE_AUTORELEASING = YES;
@@ -4644,8 +4649,9 @@
 				LD_RUNPATH_SEARCH_PATHS = "@executable_path/../Frameworks";
 				OTHER_CPLUSPLUSFLAGS = (
 					"$(inherited)",
-					"-fmodules",
-					"-fcxx-modules",
+					"-fno-modules",
+					"-Xclang",
+					"-fno-cxx-modules",
 				);
 				PRODUCT_NAME = Transmission;
 				SYSTEM_HEADER_SEARCH_PATHS = (
@@ -4662,7 +4668,7 @@
 			buildSettings = {
 				ALWAYS_SEARCH_USER_PATHS = NO;
 				CLANG_ANALYZER_LOCALIZABILITY_NONLOCALIZED = YES;
-				CLANG_CXX_LANGUAGE_STANDARD = "c++17";
+				CLANG_CXX_LANGUAGE_STANDARD = "c++20";
 				CLANG_ENABLE_OBJC_ARC = YES;
 				CLANG_ENABLE_OBJC_WEAK = YES;
 				CLANG_WARN_BLOCK_CAPTURE_AUTORELEASING = YES;
@@ -4765,7 +4771,7 @@
 			buildSettings = {
 				ALWAYS_SEARCH_USER_PATHS = NO;
 				CLANG_ANALYZER_LOCALIZABILITY_NONLOCALIZED = YES;
-				CLANG_CXX_LANGUAGE_STANDARD = "c++17";
+				CLANG_CXX_LANGUAGE_STANDARD = "c++20";
 				CLANG_ENABLE_OBJC_ARC = YES;
 				CLANG_ENABLE_OBJC_WEAK = YES;
 				CLANG_WARN_BLOCK_CAPTURE_AUTORELEASING = YES;
@@ -4849,8 +4855,9 @@
 				LD_RUNPATH_SEARCH_PATHS = "@executable_path/../Frameworks";
 				OTHER_CPLUSPLUSFLAGS = (
 					"$(inherited)",
-					"-fmodules",
-					"-fcxx-modules",
+					"-fno-modules",
+					"-Xclang",
+					"-fno-cxx-modules",
 				);
 				PRODUCT_NAME = Transmission;
 				SYSTEM_HEADER_SEARCH_PATHS = (
@@ -5034,8 +5041,9 @@
 				INSTALL_PATH = /Library/QuickLook;
 				OTHER_CPLUSPLUSFLAGS = (
 					"$(inherited)",
-					"-fmodules",
-					"-fcxx-modules",
+					"-fno-modules",
+					"-Xclang",
+					"-fno-cxx-modules",
 				);
 				PRODUCT_BUNDLE_IDENTIFIER = "org.m0k.transmission.${PRODUCT_NAME:rfc1034identifier}";
 				PRODUCT_NAME = "$(TARGET_NAME)";
@@ -5061,8 +5069,9 @@
 				INSTALL_PATH = /Library/QuickLook;
 				OTHER_CPLUSPLUSFLAGS = (
 					"$(inherited)",
-					"-fmodules",
-					"-fcxx-modules",
+					"-fno-modules",
+					"-Xclang",
+					"-fno-cxx-modules",
 				);
 				PRODUCT_BUNDLE_IDENTIFIER = "org.m0k.transmission.${PRODUCT_NAME:rfc1034identifier}";
 				PRODUCT_NAME = "$(TARGET_NAME)";
@@ -5088,8 +5097,9 @@
 				INSTALL_PATH = /Library/QuickLook;
 				OTHER_CPLUSPLUSFLAGS = (
 					"$(inherited)",
-					"-fmodules",
-					"-fcxx-modules",
+					"-fno-modules",
+					"-Xclang",
+					"-fno-cxx-modules",
 				);
 				PRODUCT_BUNDLE_IDENTIFIER = "org.m0k.transmission.${PRODUCT_NAME:rfc1034identifier}";
 				PRODUCT_NAME = "$(TARGET_NAME)";
diff --git a/gtk/Notify.cc b/gtk/Notify.cc
index a05476c96..9ce932cb8 100644
--- a/gtk/Notify.cc
+++ b/gtk/Notify.cc
@@ -53,6 +53,7 @@ template<typename... Ts>
 Glib::VariantContainerBase make_variant_tuple(Ts&&... args)
 {
     return Glib::VariantContainerBase::create_tuple(
+        // TODO(c++20): use std::remove_cvref_t (P0550R2) when GCC >= 9.1
         { Glib::Variant<std::remove_cv_t<std::remove_reference_t<Ts>>>::create(std::forward<Ts>(args))... });
 }
 
diff --git a/gtk/Torrent.cc b/gtk/Torrent.cc
index 170e2ed0e..84358f105 100644
--- a/gtk/Torrent.cc
+++ b/gtk/Torrent.cc
@@ -40,8 +40,6 @@ Glib::Value<T>& column_value_cast(Glib::ValueBase& value, Gtk::TreeModelColumn<T
 template<typename T, typename U, typename = std::enable_if_t<!std::is_floating_point_v<T>>>
 void update_cache_value(T& value, U&& new_value, Torrent::ChangeFlags& changes, Torrent::ChangeFlag flag)
 {
-    using std::rel_ops::operator!=;
-
     if (value != new_value)
     {
         value = std::forward<U>(new_value);
diff --git a/libtransmission/.clang-tidy b/libtransmission/.clang-tidy
index 7f202edc2..cf4cb67dd 100644
--- a/libtransmission/.clang-tidy
+++ b/libtransmission/.clang-tidy
@@ -3,7 +3,15 @@ HeaderFilterRegex: .*/libtransmission/.*
 
 # TODO: Enable `portability-template-virtual-member-function` after https://github.com/llvm/llvm-project/issues/139031 is fixed
 # TODO: Enable `cppcoreguidelines-pro-bounds-pointer-arithmetic` after converting all pointers to std::span
+# TODO: Enable `modernize-use-constraints` after GCC >= 10, clang 10
+# TODO: Enable `modernize-use-integer-sign-comparison` (P0586R2) after GCC >= 10.1, clang >= 13
 # PRs welcome to fix & re-enable any of these explicitly-disabled checks
+#
+# -modernize-use-ranges: GCC 10.1, clang 15
+# -modernize-loop-convert (P0896R4): GCC 10.1, clang 15
+# -readability-container-contains (P0458R2): GCC 9.1, clang 13
+# Ref: https://gcc.gnu.org/onlinedocs/libstdc++/manual/status.html
+# Ref: https://libcxx.llvm.org/Status/Cxx20.html
 Checks: >
   bugprone-*,
   -bugprone-branch-clone,
@@ -36,12 +44,18 @@ Checks: >
   -misc-no-recursion,
   -misc-non-private-member-variables-in-classes,
   modernize-*,
+  -modernize-loop-convert,
+  -modernize-use-constraints,
+  -modernize-use-designated-initializers,
+  -modernize-use-integer-sign-comparison,
+  -modernize-use-ranges,
   -modernize-use-trailing-return-type,
   performance-*,
   -performance-move-const-arg,
   portability-*,
   -portability-template-virtual-member-function,
   readability-*,
+  -readability-container-contains,
   -readability-enum-initial-value,
   -readability-function-cognitive-complexity,
   -readability-identifier-length,
diff --git a/libtransmission/announce-list.h b/libtransmission/announce-list.h
index 4ef1a5d69..b32bd2075 100644
--- a/libtransmission/announce-list.h
+++ b/libtransmission/announce-list.h
@@ -82,19 +82,19 @@ public:
         return std::size(trackers_);
     }
 
-    [[nodiscard]] TR_CONSTEXPR20 tracker_info const& at(size_t i) const
+    [[nodiscard]] TR_CONSTEXPR_VEC tracker_info const& at(size_t i) const
     {
         return trackers_.at(i);
     }
 
     [[nodiscard]] tr_tracker_tier_t nextTier() const;
 
-    [[nodiscard]] TR_CONSTEXPR20 bool operator==(tr_announce_list const& that) const
+    [[nodiscard]] TR_CONSTEXPR_VEC bool operator==(tr_announce_list const& that) const
     {
         return trackers_ == that.trackers_;
     }
 
-    [[nodiscard]] bool operator!=(tr_announce_list const& that) const
+    [[nodiscard]] TR_CONSTEXPR_VEC bool operator!=(tr_announce_list const& that) const
     {
         return trackers_ != that.trackers_;
     }
@@ -113,7 +113,7 @@ public:
     bool replace(tr_tracker_id_t id, std::string_view announce_url_sv);
     size_t set(char const* const* announce_urls, tr_tracker_tier_t const* tiers, size_t n);
 
-    TR_CONSTEXPR20 void clear()
+    TR_CONSTEXPR_VEC void clear()
     {
         trackers_.clear();
     }
diff --git a/libtransmission/announcer.cc b/libtransmission/announcer.cc
index 36551fc41..015fdce49 100644
--- a/libtransmission/announcer.cc
+++ b/libtransmission/announcer.cc
@@ -926,22 +926,22 @@ void on_announce_error(tr_tier* tier, char const* err, tr_announce_event e, time
     auto const* const current_tracker = tier->currentTracker();
     TR_ASSERT(current_tracker != nullptr);
 
-    auto req = tr_announce_request{};
-    req.port = announcer->session->advertisedPeerPort();
-    req.announce_url = current_tracker->announce_url;
-    req.tracker_id = current_tracker->tracker_id;
-    req.info_hash = tor->info_hash();
-    req.peer_id = tor->peer_id();
-    req.up = tier->byteCounts[TR_ANN_UP];
-    req.down = tier->byteCounts[TR_ANN_DOWN];
-    req.corrupt = tier->byteCounts[TR_ANN_CORRUPT];
-    req.leftUntilComplete = tor->has_metainfo() ? tor->total_size() - tor->has_total() : INT64_MAX;
-    req.event = event;
-    req.numwant = event == TR_ANNOUNCE_EVENT_STOPPED ? 0 : Numwant;
-    req.key = tor->announce_key();
-    req.partial_seed = tor->is_partial_seed();
-    req.log_name = tier->buildLogName();
-    return req;
+    return {
+        .event = event,
+        .partial_seed = tor->is_partial_seed(),
+        .port = announcer->session->advertisedPeerPort(),
+        .key = tor->announce_key(),
+        .numwant = event == TR_ANNOUNCE_EVENT_STOPPED ? 0 : Numwant,
+        .up = tier->byteCounts[TR_ANN_UP],
+        .down = tier->byteCounts[TR_ANN_DOWN],
+        .corrupt = tier->byteCounts[TR_ANN_CORRUPT],
+        .leftUntilComplete = tor->has_metainfo() ? tor->total_size() - tor->has_total() : INT64_MAX,
+        .announce_url = current_tracker->announce_url,
+        .tracker_id = current_tracker->tracker_id,
+        .peer_id = tor->peer_id(),
+        .info_hash = tor->info_hash(),
+        .log_name = tier->buildLogName(),
+    };
 }
 
 [[nodiscard]] tr_tier* getTier(tr_announcer_impl* announcer, tr_sha1_digest_t const& info_hash, size_t tier_id)
diff --git a/libtransmission/api-compat.cc b/libtransmission/api-compat.cc
index e51cac7de..b429f6b1c 100644
--- a/libtransmission/api-compat.cc
+++ b/libtransmission/api-compat.cc
@@ -653,6 +653,7 @@ void convert_keys(tr_variant& var, State& state)
     var.visit(
         [&state](auto& val)
         {
+            // TODO(c++20): use std::remove_cvref_t (P0550R2) when GCC >= 9.1
             using ValueType = std::decay_t<decltype(val)>;
 
             if constexpr (std::is_same_v<ValueType, std::string> || std::is_same_v<ValueType, std::string_view>)
diff --git a/libtransmission/error.h b/libtransmission/error.h
index 586a15c07..a08cec913 100644
--- a/libtransmission/error.h
+++ b/libtransmission/error.h
@@ -14,9 +14,9 @@
 struct tr_error
 {
 public:
-    tr_error() = default;
+    TR_CONSTEXPR_STR tr_error() = default;
 
-    tr_error(int code, std::string message)
+    TR_CONSTEXPR_STR tr_error(int code, std::string message)
         : message_{ std::move(message) }
         , code_{ code }
     {
@@ -27,7 +27,7 @@ public:
         return code_;
     }
 
-    [[nodiscard]] TR_CONSTEXPR20 auto message() const noexcept
+    [[nodiscard]] TR_CONSTEXPR_STR auto message() const noexcept
     {
         return std::string_view{ message_ };
     }
@@ -42,24 +42,24 @@ public:
         return has_value();
     }
 
-    void set(int code, std::string&& message)
+    TR_CONSTEXPR_STR void set(int code, std::string&& message)
     {
         code_ = code;
         message_ = std::move(message);
     }
 
-    void set(int code, std::string_view message)
+    TR_CONSTEXPR_STR void set(int code, std::string_view message)
     {
         code_ = code;
         message_.assign(message);
     }
 
-    void set(int code, char const* const message)
+    TR_CONSTEXPR_STR void set(int code, char const* const message)
     {
         set(code, std::string_view{ message != nullptr ? message : "" });
     }
 
-    void prefix_message(std::string_view prefix)
+    TR_CONSTEXPR_STR void prefix_message(std::string_view prefix)
     {
         message_.insert(std::begin(message_), std::begin(prefix), std::end(prefix));
     }
diff --git a/libtransmission/file-piece-map.h b/libtransmission/file-piece-map.h
index c8b5cf240..12eb93f92 100644
--- a/libtransmission/file-piece-map.h
+++ b/libtransmission/file-piece-map.h
@@ -45,7 +45,7 @@ public:
     explicit tr_file_piece_map(tr_torrent_metainfo const& tm);
     tr_file_piece_map(tr_block_info const& block_info, uint64_t const* file_sizes, size_t n_files);
 
-    [[nodiscard]] TR_CONSTEXPR20 piece_span_t piece_span_for_file(tr_file_index_t const file) const noexcept
+    [[nodiscard]] TR_CONSTEXPR_VEC piece_span_t piece_span_for_file(tr_file_index_t const file) const noexcept
     {
         return file_pieces_[file];
     }
@@ -59,7 +59,7 @@ public:
         return std::size(file_pieces_);
     }
 
-    [[nodiscard]] TR_CONSTEXPR20 auto byte_span_for_file(tr_file_index_t const file) const
+    [[nodiscard]] TR_CONSTEXPR_VEC auto byte_span_for_file(tr_file_index_t const file) const
     {
         auto const& span = file_bytes_[file];
         return tr_byte_span_t{ span.begin, span.end };
@@ -84,7 +84,7 @@ private:
 class tr_file_priorities
 {
 public:
-    TR_CONSTEXPR20 explicit tr_file_priorities(tr_file_piece_map const* fpm) noexcept
+    TR_CONSTEXPR_VEC explicit tr_file_priorities(tr_file_piece_map const* fpm) noexcept
         : fpm_{ fpm }
     {
     }
diff --git a/libtransmission/magnet-metainfo.h b/libtransmission/magnet-metainfo.h
index 09e238b7a..d9f0ea67b 100644
--- a/libtransmission/magnet-metainfo.h
+++ b/libtransmission/magnet-metainfo.h
@@ -40,7 +40,7 @@ public:
         return std::size(webseed_urls_);
     }
 
-    [[nodiscard]] TR_CONSTEXPR20 auto const& webseed(size_t i) const
+    [[nodiscard]] TR_CONSTEXPR_VEC auto const& webseed(size_t i) const
     {
         return webseed_urls_.at(i);
     }
diff --git a/libtransmission/makemeta.h b/libtransmission/makemeta.h
index 98d3cffd5..f4cfbf631 100644
--- a/libtransmission/makemeta.h
+++ b/libtransmission/makemeta.h
@@ -127,7 +127,7 @@ public:
         return files_.file_count();
     }
 
-    [[nodiscard]] TR_CONSTEXPR20 auto file_size(tr_file_index_t i) const noexcept
+    [[nodiscard]] TR_CONSTEXPR_VEC auto file_size(tr_file_index_t i) const noexcept
     {
         return files_.file_size(i);
     }
@@ -142,7 +142,7 @@ public:
         return tr_sys_path_basename(top_);
     }
 
-    [[nodiscard]] auto const& path(tr_file_index_t i) const noexcept
+    [[nodiscard]] TR_CONSTEXPR_VEC auto const& path(tr_file_index_t i) const noexcept
     {
         return files_.path(i);
     }
diff --git a/libtransmission/net.cc b/libtransmission/net.cc
index 6ea5591b1..c8d1592a0 100644
--- a/libtransmission/net.cc
+++ b/libtransmission/net.cc
@@ -665,7 +665,7 @@ std::optional<tr_address> tr_address::from_ipv4_mapped() const noexcept
 
 // --- tr_socket_addrses
 
-std::string tr_socket_address::display_name(tr_address const& address, tr_port port) noexcept
+std::string tr_socket_address::display_name(tr_address const& address, tr_port port)
 {
     return fmt::format(fmt::runtime(address.is_ipv6() ? "[{:s}]:{:d}" : "{:s}:{:d}"), address.display_name(), port.host());
 }
diff --git a/libtransmission/net.h b/libtransmission/net.h
index 40a8179cf..085b2f3af 100644
--- a/libtransmission/net.h
+++ b/libtransmission/net.h
@@ -427,9 +427,9 @@ struct tr_address
         switch (type)
         {
         case TR_AF_INET:
-            return tr_address{ TR_AF_INET, { { { { INADDR_ANY } } } } };
+            return tr_address{ TR_AF_INET, { .addr4 = { INADDR_ANY } } };
         case TR_AF_INET6:
-            return tr_address{ TR_AF_INET6, { IN6ADDR_ANY_INIT } };
+            return tr_address{ TR_AF_INET6, { .addr6 = IN6ADDR_ANY_INIT } };
         default:
             TR_ASSERT_MSG(false, "invalid type");
             return tr_address{};
@@ -472,8 +472,8 @@ struct tr_socket_address
         return port_;
     }
 
-    [[nodiscard]] static std::string display_name(tr_address const& address, tr_port port) noexcept;
-    [[nodiscard]] auto display_name() const noexcept
+    [[nodiscard]] static std::string display_name(tr_address const& address, tr_port port);
+    [[nodiscard]] auto display_name() const
     {
         return display_name(address_, port_);
     }
diff --git a/libtransmission/peer-mgr-wishlist.cc b/libtransmission/peer-mgr-wishlist.cc
index 3051022d5..ca85b4e83 100644
--- a/libtransmission/peer-mgr-wishlist.cc
+++ b/libtransmission/peer-mgr-wishlist.cc
@@ -23,7 +23,7 @@
 
 namespace
 {
-[[nodiscard]] std::vector<tr_block_span_t> make_spans(small::vector<tr_block_index_t> const& blocks)
+[[nodiscard]] TR_CONSTEXPR_VEC std::vector<tr_block_span_t> make_spans(small::vector<tr_block_index_t> const& blocks)
 {
     if (std::empty(blocks))
     {
@@ -34,7 +34,7 @@ namespace
     spans.reserve(std::size(blocks));
     for (auto span_begin = std::begin(blocks), end = std::end(blocks); span_begin != end;)
     {
-        static auto constexpr NotAdjacent = [](tr_block_index_t const lhs, tr_block_index_t const rhs)
+        auto constexpr NotAdjacent = [](tr_block_index_t const lhs, tr_block_index_t const rhs)
         {
             return lhs + 1U != rhs;
         };
@@ -218,7 +218,7 @@ private:
         }
     }
 
-    TR_CONSTEXPR20 void reset_blocks_bitfield(tr_bitfield const& requests)
+    TR_CONSTEXPR_VEC void reset_blocks_bitfield(tr_bitfield const& requests)
     {
         for (auto& candidate : candidates_)
         {
@@ -253,7 +253,7 @@ private:
 
     // ---
 
-    TR_CONSTEXPR20 void peer_disconnect(tr_bitfield const& have, tr_bitfield const& requests)
+    TR_CONSTEXPR_VEC void peer_disconnect(tr_bitfield const& have, tr_bitfield const& requests)
     {
         dec_replication_bitfield(have);
         reset_blocks_bitfield(requests);
diff --git a/libtransmission/peer-mgr.cc b/libtransmission/peer-mgr.cc
index f86e7c831..c76ddd559 100644
--- a/libtransmission/peer-mgr.cc
+++ b/libtransmission/peer-mgr.cc
@@ -15,7 +15,6 @@
 #include <memory>
 #include <optional>
 #include <tuple> // std::tie
-#include <type_traits>
 #include <unordered_map>
 #include <utility>
 #include <vector>
@@ -76,11 +75,11 @@ private:
         }
 
         return TorrentInfo{
-            tor->info_hash(), // info_hash
-            tor->peer_id(), // client_peer_id
-            tor->id(), // id
-            tor->is_done(), // is_done
-            tor->is_running() // is_running
+            .info_hash = tor->info_hash(),
+            .client_peer_id = tor->peer_id(),
+            .id = tor->id(),
+            .is_done = tor->is_done(),
+            .is_running = tor->is_running(),
         };
     }
 
@@ -308,8 +307,8 @@ void tr_peer_info::update_canonical_priority()
     // FFFF:FFFF:FFFF:FFFF:5555:5555:5555:5555, etc...
     static auto constexpr MaskStartBaseOffset = std::array{ 2U, 6U };
     auto const base_idx = MaskStartBaseOffset[type];
-    auto const mismatch_idx = std::mismatch(first, second, second).first - first;
-    for (auto i = mismatch_idx >= base_idx ? mismatch_idx + 1 : base_idx; i < address_size; ++i)
+    auto const mismatch_idx = static_cast<size_t>(std::mismatch(first, second, second).first - first);
+    for (auto i = mismatch_idx >= base_idx ? mismatch_idx + 1U : base_idx; i < address_size; ++i)
     {
         first[i] &= std::byte{ 0x55 };
         second[i] &= std::byte{ 0x55 };
@@ -354,8 +353,7 @@ constexpr struct
     }
 
     template<typename T>
-    [[nodiscard]] constexpr std::enable_if_t<std::is_same_v<std::decay_t<decltype(*std::declval<T>())>, tr_peer_info>, bool>
-    operator()(T const& a, T const& b) const noexcept
+    [[nodiscard]] constexpr bool operator()(T const& a, T const& b) const noexcept
     {
         return compare(*a, *b) < 0;
     }
@@ -2460,9 +2458,7 @@ struct ComparePeerInfo
     }
 
     template<typename T>
-    [[nodiscard]] std::enable_if_t<std::is_same_v<std::decay_t<decltype(*std::declval<T>())>, tr_peer_info>, bool> operator()(
-        T const& a,
-        T const& b) const noexcept
+    [[nodiscard]] bool operator()(T const& a, T const& b) const noexcept
     {
         return compare(*a, *b) < 0;
     }
diff --git a/libtransmission/rpcimpl.cc b/libtransmission/rpcimpl.cc
index fce79e5e6..e0c906d8f 100644
--- a/libtransmission/rpcimpl.cc
+++ b/libtransmission/rpcimpl.cc
@@ -2606,6 +2606,9 @@ using SessionAccessors = std::pair<SessionGetter, SessionSetter>;
         [](tr_session const& /*src*/) -> tr_variant { return tr_variant::unmanaged_string(LONG_VERSION_STRING); },
         nullptr);
 
+    // `row` could have been replaced by structured bindings,
+    // but it's not available until clang 16
+    // https://github.com/llvm/llvm-project/commit/44f2baa3804a62ca793f0ff3e43aa71cea91a795
     for (auto const& row : tr_session::Scripts)
     {
         auto const script = row.script;
diff --git a/libtransmission/serializer.h b/libtransmission/serializer.h
index 669174ca6..46f69be84 100644
--- a/libtransmission/serializer.h
+++ b/libtransmission/serializer.h
@@ -31,6 +31,7 @@ namespace detail
 // NOLINTBEGIN(readability-identifier-naming)
 // use std-style naming for these traits
 
+// TODO(c++20): use std::remove_cvref_t (P0550R2) when GCC >= 9.1
 template<typename T>
 using remove_cvref_t = std::remove_cv_t<std::remove_reference_t<T>>;
 
diff --git a/libtransmission/session-alt-speeds.cc b/libtransmission/session-alt-speeds.cc
index f8002e615..ea14e41f0 100644
--- a/libtransmission/session-alt-speeds.cc
+++ b/libtransmission/session-alt-speeds.cc
@@ -77,7 +77,7 @@ void tr_session_alt_speeds::set_active(bool active, ChangeReason reason, bool fo
     }
 }
 
-[[nodiscard]] bool tr_session_alt_speeds::is_active_minute(time_t time) const noexcept
+[[nodiscard]] bool tr_session_alt_speeds::is_active_minute(time_t time) const
 {
     auto const tm = *std::localtime(&time);
 
diff --git a/libtransmission/session-alt-speeds.h b/libtransmission/session-alt-speeds.h
index 624a67a32..e7bf057de 100644
--- a/libtransmission/session-alt-speeds.h
+++ b/libtransmission/session-alt-speeds.h
@@ -189,7 +189,7 @@ private:
     void set_active(bool active, ChangeReason reason, bool force);
 
     // whether `time` hits in one of the `minutes_` that is true
-    [[nodiscard]] bool is_active_minute(time_t time) const noexcept;
+    [[nodiscard]] bool is_active_minute(time_t time) const;
 
     static int constexpr MinutesPerHour = 60;
     static int constexpr MinutesPerDay = MinutesPerHour * 24;
diff --git a/libtransmission/session-id.cc b/libtransmission/session-id.cc
index 71f8398e1..d319058a3 100644
--- a/libtransmission/session-id.cc
+++ b/libtransmission/session-id.cc
@@ -110,13 +110,14 @@ tr_session_id::session_id_t tr_session_id::make_session_id()
     return session_id;
 }
 
+// NOLINTNEXTLINE(bugprone-exception-escape)
 tr_session_id::~tr_session_id()
 {
     destroy_lockfile(current_lock_file_, std::data(current_value_));
     destroy_lockfile(previous_lock_file_, std::data(previous_value_));
 }
 
-bool tr_session_id::is_local(std::string_view session_id) noexcept
+bool tr_session_id::is_local(std::string_view session_id)
 {
     if (std::empty(session_id))
     {
@@ -158,7 +159,7 @@ bool tr_session_id::is_local(std::string_view session_id) noexcept
     return is_local;
 }
 
-std::string_view tr_session_id::sv() const noexcept
+std::string_view tr_session_id::sv() const
 {
     if (auto const now = get_current_time_(); now >= expires_at_)
     {
@@ -174,7 +175,7 @@ std::string_view tr_session_id::sv() const noexcept
     return std::string_view{ std::data(current_value_), std::size(current_value_) - 1 };
 }
 
-char const* tr_session_id::c_str() const noexcept
+char const* tr_session_id::c_str() const
 {
     return std::data(sv()); // current_value_ is zero-terminated
 }
diff --git a/libtransmission/session-id.h b/libtransmission/session-id.h
index 3309b5c5c..eead896d0 100644
--- a/libtransmission/session-id.h
+++ b/libtransmission/session-id.h
@@ -36,11 +36,11 @@ public:
      * relative paths to absolute before passing through RPC, or presenting
      * different UI for local and remote sessions.
      */
-    [[nodiscard]] static bool is_local(std::string_view session_id) noexcept;
+    [[nodiscard]] static bool is_local(std::string_view session_id);
 
     // current session identifier
-    [[nodiscard]] std::string_view sv() const noexcept;
-    [[nodiscard]] char const* c_str() const noexcept;
+    [[nodiscard]] std::string_view sv() const;
+    [[nodiscard]] char const* c_str() const;
 
 private:
     static auto constexpr SessionIdSize = size_t{ 48 };
diff --git a/libtransmission/session.h b/libtransmission/session.h
index 469883b3f..85316239f 100644
--- a/libtransmission/session.h
+++ b/libtransmission/session.h
@@ -555,7 +555,7 @@ public:
 
     explicit tr_session(std::string_view config_dir, tr_variant const& settings_dict);
 
-    [[nodiscard]] std::string_view sessionId() const noexcept
+    [[nodiscard]] std::string_view sessionId() const
     {
         return session_id_.sv();
     }
diff --git a/libtransmission/torrent-files.h b/libtransmission/torrent-files.h
index 9b3244402..671d42af1 100644
--- a/libtransmission/torrent-files.h
+++ b/libtransmission/torrent-files.h
@@ -40,7 +40,7 @@ public:
         return std::size(files_);
     }
 
-    [[nodiscard]] TR_CONSTEXPR20 uint64_t file_size(tr_file_index_t file_index) const
+    [[nodiscard]] TR_CONSTEXPR_VEC uint64_t file_size(tr_file_index_t file_index) const
     {
         return files_.at(file_index).size_;
     }
@@ -50,7 +50,7 @@ public:
         return total_size_;
     }
 
-    [[nodiscard]] TR_CONSTEXPR20 std::string const& path(tr_file_index_t file_index) const
+    [[nodiscard]] TR_CONSTEXPR_VEC std::string const& path(tr_file_index_t file_index) const
     {
         return files_.at(file_index).path_;
     }
@@ -71,17 +71,17 @@ public:
         }
     }
 
-    void reserve(size_t n_files)
+    TR_CONSTEXPR_VEC void reserve(size_t n_files)
     {
         files_.reserve(n_files);
     }
 
-    void shrink_to_fit()
+    TR_CONSTEXPR_VEC void shrink_to_fit()
     {
         files_.shrink_to_fit();
     }
 
-    TR_CONSTEXPR20 void clear() noexcept
+    TR_CONSTEXPR_VEC void clear() noexcept
     {
         files_.clear();
         total_size_ = uint64_t{};
@@ -102,7 +102,7 @@ public:
         return ret;
     }
 
-    tr_file_index_t add(std::string_view path, uint64_t file_size)
+    TR_CONSTEXPR_VEC tr_file_index_t add(std::string_view path, uint64_t file_size)
     {
         auto const ret = static_cast<tr_file_index_t>(std::size(files_));
         files_.emplace_back(path, file_size);
@@ -176,7 +176,7 @@ private:
     struct file_t
     {
     public:
-        void set_path(std::string_view subpath)
+        TR_CONSTEXPR_STR void set_path(std::string_view subpath)
         {
             if (path_ != subpath)
             {
@@ -185,7 +185,7 @@ private:
             }
         }
 
-        file_t(std::string_view path, uint64_t size)
+        TR_CONSTEXPR_STR file_t(std::string_view path, uint64_t size)
             : path_{ path }
             , size_{ size }
         {
diff --git a/libtransmission/torrent-magnet.h b/libtransmission/torrent-magnet.h
index 84ba0ca7e..9f24322e0 100644
--- a/libtransmission/torrent-magnet.h
+++ b/libtransmission/torrent-magnet.h
@@ -48,7 +48,7 @@ public:
         return metadata_;
     }
 
-    [[nodiscard]] TR_CONSTEXPR20 std::string_view log_name() const noexcept
+    [[nodiscard]] TR_CONSTEXPR_STR std::string_view log_name() const noexcept
     {
         return log_name_;
     }
diff --git a/libtransmission/torrent-metainfo.h b/libtransmission/torrent-metainfo.h
index 0c7c9ffe5..a3e3569fb 100644
--- a/libtransmission/torrent-metainfo.h
+++ b/libtransmission/torrent-metainfo.h
@@ -46,11 +46,11 @@ public:
     {
         return files().file_count();
     }
-    [[nodiscard]] TR_CONSTEXPR20 auto file_size(tr_file_index_t i) const
+    [[nodiscard]] TR_CONSTEXPR_VEC auto file_size(tr_file_index_t i) const
     {
         return files().file_size(i);
     }
-    [[nodiscard]] TR_CONSTEXPR20 auto const& file_subpath(tr_file_index_t i) const
+    [[nodiscard]] TR_CONSTEXPR_VEC auto const& file_subpath(tr_file_index_t i) const
     {
         return files().path(i);
     }
@@ -132,7 +132,7 @@ public:
         return is_private_;
     }
 
-    [[nodiscard]] TR_CONSTEXPR20 tr_sha1_digest_t const& piece_hash(tr_piece_index_t piece) const
+    [[nodiscard]] TR_CONSTEXPR_VEC tr_sha1_digest_t const& piece_hash(tr_piece_index_t piece) const
     {
         return pieces_[piece];
     }
diff --git a/libtransmission/torrent.cc b/libtransmission/torrent.cc
index 17a6c2f0f..712bfb206 100644
--- a/libtransmission/torrent.cc
+++ b/libtransmission/torrent.cc
@@ -1434,11 +1434,29 @@ tr_file_view tr_torrentFile(tr_torrent const* tor, tr_file_index_t file)
 
     if (tor->is_seed() || length == 0)
     {
-        return { subpath.c_str(), length, length, 1.0, begin, end, priority, wanted };
+        return {
+            .name = subpath.c_str(),
+            .have = length,
+            .length = length,
+            .progress = 1.0,
+            .beginPiece = begin,
+            .endPiece = end,
+            .priority = priority,
+            .wanted = wanted,
+        };
     }
 
     auto const have = tor->completion_.count_has_bytes_in_span(tor->byte_span_for_file(file));
-    return { subpath.c_str(), have, length, have >= length ? 1.0 : have / double(length), begin, end, priority, wanted };
+    return {
+        .name = subpath.c_str(),
+        .have = have,
+        .length = length,
+        .progress = have >= length ? 1.0 : have / double(length),
+        .beginPiece = begin,
+        .endPiece = end,
+        .priority = priority,
+        .wanted = wanted,
+    };
 }
 
 size_t tr_torrentFileCount(tr_torrent const* torrent)
@@ -1473,21 +1491,19 @@ size_t tr_torrentTrackerCount(tr_torrent const* tor)
 tr_torrent_view tr_torrentView(tr_torrent const* tor)
 {
     TR_ASSERT(tr_isTorrent(tor));
-
-    auto ret = tr_torrent_view{};
-    ret.name = tor->name().c_str();
-    ret.hash_string = tor->info_hash_string().c_str();
-    ret.comment = tor->comment().c_str();
-    ret.creator = tor->creator().c_str();
-    ret.source = tor->source().c_str();
-    ret.total_size = tor->total_size();
-    ret.date_created = tor->date_created();
-    ret.piece_size = tor->piece_size();
-    ret.n_pieces = tor->piece_count();
-    ret.is_private = tor->is_private();
-    ret.is_folder = tor->file_count() > 1 || (tor->file_count() == 1 && tr_strv_contains(tor->file_subpath(0), '/'));
-
-    return ret;
+    return {
+        .name = tor->name().c_str(),
+        .hash_string = tor->info_hash_string().c_str(),
+        .comment = tor->comment().c_str(),
+        .creator = tor->creator().c_str(),
+        .source = tor->source().c_str(),
+        .total_size = tor->total_size(),
+        .date_created = tor->date_created(),
+        .piece_size = tor->piece_size(),
+        .n_pieces = tor->piece_count(),
+        .is_private = tor->is_private(),
+        .is_folder = tor->file_count() > 1 || (tor->file_count() == 1 && tr_strv_contains(tor->file_subpath(0), '/')),
+    };
 }
 
 std::string tr_torrentFilename(tr_torrent const* tor)
diff --git a/libtransmission/torrent.h b/libtransmission/torrent.h
index 2cef6059d..0e3147f8d 100644
--- a/libtransmission/torrent.h
+++ b/libtransmission/torrent.h
@@ -480,12 +480,12 @@ struct tr_torrent
         return metainfo_.file_count();
     }
 
-    [[nodiscard]] TR_CONSTEXPR20 auto const& file_subpath(tr_file_index_t i) const
+    [[nodiscard]] TR_CONSTEXPR_VEC auto const& file_subpath(tr_file_index_t i) const
     {
         return metainfo_.file_subpath(i);
     }
 
-    [[nodiscard]] TR_CONSTEXPR20 auto file_size(tr_file_index_t i) const
+    [[nodiscard]] TR_CONSTEXPR_VEC auto file_size(tr_file_index_t i) const
     {
         return metainfo_.file_size(i);
     }
@@ -517,7 +517,7 @@ struct tr_torrent
         return metainfo_.webseed_count();
     }
 
-    [[nodiscard]] TR_CONSTEXPR20 auto const& webseed(size_t i) const
+    [[nodiscard]] TR_CONSTEXPR_VEC auto const& webseed(size_t i) const
     {
         return metainfo_.webseed(i);
     }
@@ -1272,7 +1272,7 @@ private:
     // must be called after the torrent's announce list changes.
     void on_announce_list_changed();
 
-    [[nodiscard]] TR_CONSTEXPR20 tr_byte_span_t byte_span_for_file(tr_file_index_t file) const
+    [[nodiscard]] TR_CONSTEXPR_VEC tr_byte_span_t byte_span_for_file(tr_file_index_t file) const
     {
         return fpm_.byte_span_for_file(file);
     }
diff --git a/libtransmission/tr-buffer.h b/libtransmission/tr-buffer.h
index 70da6fefe..c09ce4924 100644
--- a/libtransmission/tr-buffer.h
+++ b/libtransmission/tr-buffer.h
@@ -111,7 +111,7 @@ public:
             return {};
         }
 
-        if (auto const n_sent = send(sockfd, reinterpret_cast<char const*>(data()), n_bytes, 0); n_sent >= 0U)
+        if (auto const n_sent = send(sockfd, reinterpret_cast<char const*>(data()), n_bytes, 0); n_sent >= 0)
         {
             drain(n_sent);
             return n_sent;
diff --git a/libtransmission/tr-macros.h b/libtransmission/tr-macros.h
index 8196bf20a..d85dff8d8 100644
--- a/libtransmission/tr-macros.h
+++ b/libtransmission/tr-macros.h
@@ -17,6 +17,18 @@
 #define TR_CONSTEXPR20
 #endif
 
+#if __cpp_lib_constexpr_vector >= 201907L
+#define TR_CONSTEXPR_VEC constexpr
+#else
+#define TR_CONSTEXPR_VEC
+#endif
+
+#if __cpp_lib_constexpr_string >= 201907L
+#define TR_CONSTEXPR_STR constexpr
+#else
+#define TR_CONSTEXPR_STR
+#endif
+
 #if __cplusplus >= 202302L // _MSVC_LANG value for C++23 not available yet
 #define TR_CONSTEXPR23 constexpr
 #else
diff --git a/libtransmission/utils.h b/libtransmission/utils.h
index 1a285426c..4fd5e6abd 100644
--- a/libtransmission/utils.h
+++ b/libtransmission/utils.h
@@ -114,33 +114,41 @@ template<typename T>
  */
 [[nodiscard]] bool tr_wildmat(char const* text, char const* pattern);
 
+// c++23 (P1679R3), GCC 11.1, clang 12
 template<typename T>
-[[nodiscard]] constexpr bool tr_strv_contains(std::string_view sv, T key) noexcept // c++23
+[[nodiscard]] constexpr bool tr_strv_contains(std::string_view sv, T key) noexcept
 {
     return sv.find(key) != std::string_view::npos;
 }
 
-[[nodiscard]] constexpr bool tr_strv_starts_with(std::string_view sv, char key) // c++20
+// c++20 (P0457R2), GCC 9.1, clang 6
+[[nodiscard]] constexpr bool tr_strv_starts_with(std::string_view sv, char key)
 {
     return !std::empty(sv) && sv.front() == key;
 }
 
-[[nodiscard]] constexpr bool tr_strv_starts_with(std::string_view sv, std::string_view key) // c++20
+// c++20 (P0457R2), GCC 9.1, clang 6
+[[nodiscard]] constexpr bool tr_strv_starts_with(std::string_view sv, std::string_view key)
 {
     return std::size(key) <= std::size(sv) && sv.substr(0, std::size(key)) == key;
 }
 
-[[nodiscard]] constexpr bool tr_strv_starts_with(std::wstring_view sv, std::wstring_view key) // c++20
+// c++20 (P0457R2), GCC 9.1, clang 6
+[[nodiscard]] constexpr bool tr_strv_starts_with(
+    std::wstring_view sv,
+    std::wstring_view key) // c++20 (P0457R2), GCC 9.1, clang 6
 {
     return std::size(key) <= std::size(sv) && sv.substr(0, std::size(key)) == key;
 }
 
-[[nodiscard]] constexpr bool tr_strv_ends_with(std::string_view sv, std::string_view key) // c++20
+// c++20 (P0457R2), GCC 9.1, clang 6
+[[nodiscard]] constexpr bool tr_strv_ends_with(std::string_view sv, std::string_view key)
 {
     return std::size(key) <= std::size(sv) && sv.substr(std::size(sv) - std::size(key)) == key;
 }
 
-[[nodiscard]] constexpr bool tr_strv_ends_with(std::string_view sv, char key) // c++20
+// c++20 (P0457R2), GCC 9.1, clang 6
+[[nodiscard]] constexpr bool tr_strv_ends_with(std::string_view sv, char key)
 {
     return !std::empty(sv) && sv.back() == key;
 }
diff --git a/libtransmission/values.h b/libtransmission/values.h
index 4f0b50331..5de5e6306 100644
--- a/libtransmission/values.h
+++ b/libtransmission/values.h
@@ -48,7 +48,7 @@ struct Config
     struct Units
     {
         template<typename... Names> // NOLINTNEXTLINE(google-explicit-constructor, cppcoreguidelines-pro-type-member-init)
-        Units(Base base, Names... names) noexcept
+        Units(Base base, Names... names)
         {
             set_base(base);
 
@@ -204,7 +204,7 @@ public:
         return compare(that) >= 0;
     }
 
-    std::string_view to_string(char* buf, size_t buflen) const noexcept
+    std::string_view to_string(char* buf, size_t buflen) const
     {
         auto idx = size_t{ 0 };
         auto val = 1.0 * base_quantity_;
diff --git a/libtransmission/variant.cc b/libtransmission/variant.cc
index ba003d54b..48498c246 100644
--- a/libtransmission/variant.cc
+++ b/libtransmission/variant.cc
@@ -192,6 +192,7 @@ tr_variant& tr_variant::merge(tr_variant const& that)
     that.visit(
         [this](auto const& value)
         {
+            // TODO(c++20): use std::remove_cvref_t (P0550R2) when GCC >= 9.1
             using ValueType = std::decay_t<decltype(value)>;
 
             if constexpr (
diff --git a/libtransmission/variant.h b/libtransmission/variant.h
index 92adff07f..9b90f2288 100644
--- a/libtransmission/variant.h
+++ b/libtransmission/variant.h
@@ -100,9 +100,9 @@ public:
             return Vector::const_iterator{ const_cast<Map*>(this)->find(key) };
         }
 
-        [[nodiscard]] auto contains(tr_quark const key) const noexcept
+        [[nodiscard]] TR_CONSTEXPR20 auto contains(tr_quark const key) const noexcept
         {
-            return find(key) != end(); // NOLINT(readability-container-contains)
+            return find(key) != end();
         }
 
         [[nodiscard]] TR_CONSTEXPR20 auto size() const noexcept
@@ -131,7 +131,7 @@ public:
             return 0U;
         }
 
-        bool replace_key(tr_quark const old_key, tr_quark const new_key)
+        TR_CONSTEXPR20 bool replace_key(tr_quark const old_key, tr_quark const new_key)
         {
             if (contains(new_key))
             {
@@ -309,6 +309,7 @@ public:
     template<typename Val>
     [[nodiscard]] constexpr auto* get_if() noexcept
     {
+        // TODO(c++20): use std::remove_cvref_t (P0550R2) when GCC >= 9.1
         static_assert(
             !std::is_same_v<std::decay_t<Val>, std::string> && !std::is_same_v<std::decay_t<Val>, std::string_view>,
             "not supported -- use value_if<std::string_view>() instead.");
@@ -318,6 +319,7 @@ public:
     template<typename Val>
     [[nodiscard]] constexpr auto const* get_if() const noexcept
     {
+        // TODO(c++20): use std::remove_cvref_t (P0550R2) when GCC >= 9.1
         static_assert(
             !std::is_same_v<std::decay_t<Val>, std::string> && !std::is_same_v<std::decay_t<Val>, std::string_view>,
             "not supported -- use value_if<std::string_view>() instead.");
diff --git a/macosx/CMakeLists.txt b/macosx/CMakeLists.txt
index ce22180e4..a704e1136 100644
--- a/macosx/CMakeLists.txt
+++ b/macosx/CMakeLists.txt
@@ -9,8 +9,10 @@ find_program(CODESIGN_EXECUTABLE codesign REQUIRED)
 set(MAC_BUNDLE_NAME Transmission)
 
 add_compile_options(
-    $<$<COMPILE_LANGUAGE:C,CXX>:-fmodules>
-    $<$<COMPILE_LANGUAGE:C,CXX>:-fcxx-modules>
+    $<$<COMPILE_LANGUAGE:C>:-fmodules>
+
+    # https://github.com/llvm/llvm-project/issues/57432
+    "$<$<COMPILE_LANG_AND_ID:CXX,AppleClang,Clang>:-fno-modules;-Xclang;-fno-cxx-modules>"
 
     # equivalent of XCODE_ATTRIBUTE_CLANG_ENABLE_OBJC_ARC YES for this directory
     $<$<COMPILE_LANGUAGE:C,CXX>:-fobjc-arc>
@@ -415,6 +417,7 @@ target_link_libraries(${TR_NAME}-mac
         "-framework Quartz"
         "-framework Security"
         "-weak_framework Sparkle"
+        "-weak_framework UniformTypeIdentifiers"
         "-weak_framework UserNotifications")
 
 if(NOT CMAKE_GENERATOR STREQUAL Xcode)
diff --git a/macosx/Controller.mm b/macosx/Controller.mm
index c129a6d77..462cdfba1 100644
--- a/macosx/Controller.mm
+++ b/macosx/Controller.mm
@@ -2,10 +2,17 @@
 // It may be used under the MIT (SPDX: MIT) license.
 // License text can be found in the licenses/ folder.
 
+#if __has_feature(modules)
 @import Carbon;
 @import UserNotifications;
 
 @import Sparkle;
+#else
+#import <Carbon/Carbon.h>
+#import <UserNotifications/UserNotifications.h>
+
+#import <Sparkle/Sparkle.h>
+#endif
 
 #include <atomic> /* atomic, atomic_fetch_add_explicit, memory_order_relaxed */
 
diff --git a/macosx/ExpandedPathToIconTransformer.mm b/macosx/ExpandedPathToIconTransformer.mm
index 63b64bd49..f5186f971 100644
--- a/macosx/ExpandedPathToIconTransformer.mm
+++ b/macosx/ExpandedPathToIconTransformer.mm
@@ -2,7 +2,11 @@
 // It may be used under the MIT (SPDX: MIT) license.
 // License text can be found in the licenses/ folder.
 
+#if __has_feature(modules)
 @import AppKit;
+#else
+#import <AppKit/AppKit.h>
+#endif
 
 #import "ExpandedPathToIconTransformer.h"
 
diff --git a/macosx/FileListNode.mm b/macosx/FileListNode.mm
index 9a77477bd..ccaaea6d2 100644
--- a/macosx/FileListNode.mm
+++ b/macosx/FileListNode.mm
@@ -2,7 +2,11 @@
 // It may be used under the MIT (SPDX: MIT) license.
 // License text can be found in the licenses/ folder.
 
+#if __has_feature(modules)
 @import AppKit;
+#else
+#import <AppKit/AppKit.h>
+#endif
 
 #import "FileListNode.h"
 
diff --git a/macosx/QuickLookExtension/CMakeLists.txt b/macosx/QuickLookExtension/CMakeLists.txt
index 8043d2106..538b6d738 100644
--- a/macosx/QuickLookExtension/CMakeLists.txt
+++ b/macosx/QuickLookExtension/CMakeLists.txt
@@ -99,9 +99,11 @@ target_link_libraries(${TR_NAME}-mac-qlappex
     PRIVATE
         ${TR_NAME}
         "-framework Foundation"
+        "-framework AppKit"
         "-framework CoreFoundation"
         "-framework CoreServices"
         "-framework Quartz"
+        "-framework UniformTypeIdentifiers"
         "-framework QuickLook")
 
 install(
diff --git a/macosx/QuickLookPlugin/GeneratePreviewForURL.mm b/macosx/QuickLookPlugin/GeneratePreviewForURL.mm
index 4dd651a0b..d3d45bdbf 100644
--- a/macosx/QuickLookPlugin/GeneratePreviewForURL.mm
+++ b/macosx/QuickLookPlugin/GeneratePreviewForURL.mm
@@ -1,6 +1,12 @@
+#if __has_feature(modules)
 @import AppKit;
 @import CoreFoundation;
 @import QuickLook;
+#else
+#import <AppKit/AppKit.h>
+#import <CoreFoundation/CFPlugInCOM.h>
+#import <QuickLook/QuickLook.h>
+#endif
 
 #include <string>
 #include <unordered_map>
diff --git a/macosx/QuickLookPlugin/GenerateThumbnailForURL.mm b/macosx/QuickLookPlugin/GenerateThumbnailForURL.mm
index d97465565..934164401 100644
--- a/macosx/QuickLookPlugin/GenerateThumbnailForURL.mm
+++ b/macosx/QuickLookPlugin/GenerateThumbnailForURL.mm
@@ -1,5 +1,11 @@
+#if __has_feature(modules)
 @import CoreFoundation;
 @import QuickLook;
+#else
+#import <CoreFoundation/CoreFoundation.h>
+#import <CoreFoundation/CFPlugInCOM.h>
+#import <QuickLook/QuickLook.h>
+#endif
 
 QL_EXTERN_C_BEGIN
 OSStatus GenerateThumbnailForURL(void* thisInterface, QLThumbnailRequestRef thumbnail, CFURLRef url, CFStringRef contentTypeUTI, CFDictionaryRef options, CGSize maxSize);
diff --git a/macosx/SparkleProxy.mm b/macosx/SparkleProxy.mm
index 496f43788..5f78a0f83 100644
--- a/macosx/SparkleProxy.mm
+++ b/macosx/SparkleProxy.mm
@@ -2,8 +2,13 @@
 // It may be used under the MIT (SPDX: MIT) license.
 // License text can be found in the licenses/ folder.
 
+#if __has_feature(modules)
 @import ObjectiveC;
 @import AppKit;
+#else
+#import <objc/runtime.h>
+#import <AppKit/AppKit.h>
+#endif
 #import "NSStringAdditions.h"
 
 // Development-only proxy when app is not signed for running Sparkle
diff --git a/macosx/main.mm b/macosx/main.mm
index 396f5613b..0711709d5 100644
--- a/macosx/main.mm
+++ b/macosx/main.mm
@@ -2,7 +2,11 @@
 // It may be used under the MIT (SPDX: MIT) license.
 // License text can be found in the licenses/ folder.
 
+#if __has_feature(modules)
 @import AppKit;
+#else
+#import <AppKit/AppKit.h>
+#endif
 
 #include <libtransmission/transmission.h>
 
diff --git a/tests/libtransmission/.clang-tidy b/tests/libtransmission/.clang-tidy
index 6b2af110a..d4cea88e3 100644
--- a/tests/libtransmission/.clang-tidy
+++ b/tests/libtransmission/.clang-tidy
@@ -3,6 +3,11 @@ HeaderFilterRegex: .*/tests/libtransmission/.*
 
 # Many of these checks are disabled only because the code hasn't been
 # cleaned up yet. Pull requests welcomed.
+#
+# -modernize-loop-convert (P0896R4): GCC 10.1, clang 15
+# -readability-container-contains (P0458R2): GCC 9.1, clang 13
+# Ref: https://gcc.gnu.org/onlinedocs/libstdc++/manual/status.html
+# Ref: https://libcxx.llvm.org/Status/Cxx20.html
 Checks: >
   bugprone-*,
   -bugprone-easily-swappable-parameters,
@@ -30,9 +35,13 @@ Checks: >
   -misc-no-recursion,
   -misc-non-private-member-variables-in-classes,
   modernize-*,
+  -modernize-loop-convert,
+  -modernize-use-designated-initializers,
+  -modernize-use-ranges,
   -modernize-use-trailing-return-type,
   performance-*,
   readability-*,
+  -readability-container-contains,
   -readability-enum-initial-value,
   -readability-function-cognitive-complexity,
   -readability-identifier-length,
diff --git a/tests/libtransmission/announcer-udp-test.cc b/tests/libtransmission/announcer-udp-test.cc
index e53ad18a5..53767fb23 100644
--- a/tests/libtransmission/announcer-udp-test.cc
+++ b/tests/libtransmission/announcer-udp-test.cc
@@ -635,35 +635,38 @@ TEST_F(AnnouncerUdpTest, canAnnounceIPv4)
         { tr_address::from_string("192.168.1.3"sv).value_or(tr_address{}), tr_port::from_host(2022) },
     } };
 
-    auto request = tr_announce_request{};
-    request.event = TR_ANNOUNCE_EVENT_STARTED;
-    request.port = tr_port::from_host(80);
-    request.key = 0xCAFE;
-    request.numwant = 20;
-    request.up = 1;
-    request.down = 2;
-    request.corrupt = 3;
-    request.leftUntilComplete = 100;
-    request.announce_url = "https://127.0.0.1/announce"sv;
-    request.tracker_id = "fnord"s;
-    request.peer_id = tr_peerIdInit();
-    request.info_hash = tr_rand_obj<tr_sha1_digest_t>();
+    auto const request = tr_announce_request{
+        .event = TR_ANNOUNCE_EVENT_STARTED,
+        .port = tr_port::from_host(80),
+        .key = 0xCAFE,
+        .numwant = 20,
+        .up = 1,
+        .down = 2,
+        .corrupt = 3,
+        .leftUntilComplete = 100,
+        .announce_url = tr_interned_string{ "https://127.0.0.1/announce"sv },
+        .tracker_id = "fnord"s,
+        .peer_id = tr_peerIdInit(),
+        .info_hash = tr_rand_obj<tr_sha1_digest_t>(),
+        .log_name = {},
+    };
 
-    auto expected_response = tr_announce_response{};
-    expected_response.info_hash = request.info_hash;
-    expected_response.did_connect = true;
-    expected_response.did_timeout = false;
-    expected_response.interval = Interval;
-    expected_response.min_interval = 0; // not specified in UDP announce
-    expected_response.seeders = Seeders;
-    expected_response.leechers = Leechers;
-    expected_response.downloads = std::nullopt; // not specified in UDP announce
-    expected_response.pex = std::vector<tr_pex>{ tr_pex{ addresses[0] }, tr_pex{ addresses[1] }, tr_pex{ addresses[2] } };
-    expected_response.pex6 = {};
-    expected_response.errmsg = {};
-    expected_response.warning = {};
-    expected_response.tracker_id = {}; // not specified in UDP announce
-    expected_response.external_ip = {};
+    auto const expected_response = tr_announce_response{
+        .info_hash = request.info_hash,
+        .did_connect = true,
+        .did_timeout = false,
+        .interval = Interval,
+        .min_interval = 0, // not specified in UDP announce
+        .seeders = Seeders,
+        .leechers = Leechers,
+        .downloads = std::nullopt, // not specified in UDP announce
+        .pex = std::vector{ tr_pex{ addresses[0] }, tr_pex{ addresses[1] }, tr_pex{ addresses[2] } },
+        .pex6 = {},
+        .errmsg = {},
+        .warning = {},
+        .tracker_id = {}, // not specified in UDP announce
+        .external_ip = {},
+    };
 
     // build the announcer
     auto mediator = MockMediator{};
@@ -718,35 +721,38 @@ TEST_F(AnnouncerUdpTest, canAnnounceIPv6)
         { tr_address::from_string("fd12:3456:789a:1::3"sv).value_or(tr_address{}), tr_port::from_host(2022) },
     } };
 
-    auto request = tr_announce_request{};
-    request.event = TR_ANNOUNCE_EVENT_STARTED;
-    request.port = tr_port::from_host(80);
-    request.key = 0xCAFE;
-    request.numwant = 20;
-    request.up = 1;
-    request.down = 2;
-    request.corrupt = 3;
-    request.leftUntilComplete = 100;
-    request.announce_url = "https://[::1]/announce"sv;
-    request.tracker_id = "fnord"s;
-    request.peer_id = tr_peerIdInit();
-    request.info_hash = tr_rand_obj<tr_sha1_digest_t>();
+    auto const request = tr_announce_request{
+        .event = TR_ANNOUNCE_EVENT_STARTED,
+        .port = tr_port::from_host(80),
+        .key = 0xCAFE,
+        .numwant = 20,
+        .up = 1,
+        .down = 2,
+        .corrupt = 3,
+        .leftUntilComplete = 100,
+        .announce_url = tr_interned_string{ "https://[::1]/announce"sv },
+        .tracker_id = "fnord"s,
+        .peer_id = tr_peerIdInit(),
+        .info_hash = tr_rand_obj<tr_sha1_digest_t>(),
+        .log_name = {},
+    };
 
-    auto expected_response = tr_announce_response{};
-    expected_response.info_hash = request.info_hash;
-    expected_response.did_connect = true;
-    expected_response.did_timeout = false;
-    expected_response.interval = Interval;
-    expected_response.min_interval = 0; // not specified in UDP announce
-    expected_response.seeders = Seeders;
-    expected_response.leechers = Leechers;
-    expected_response.downloads = std::nullopt; // not specified in UDP announce
-    expected_response.pex = {};
-    expected_response.pex6 = std::vector<tr_pex>{ tr_pex{ addresses[0] }, tr_pex{ addresses[1] }, tr_pex{ addresses[2] } };
-    expected_response.errmsg = {};
-    expected_response.warning = {};
-    expected_response.tracker_id = {}; // not specified in UDP announce
-    expected_response.external_ip = {};
+    auto const expected_response = tr_announce_response{
+        .info_hash = request.info_hash,
+        .did_connect = true,
+        .did_timeout = false,
+        .interval = Interval,
+        .min_interval = 0, // not specified in UDP announce
+        .seeders = Seeders,
+        .leechers = Leechers,
+        .downloads = std::nullopt, // not specified in UDP announce
+        .pex = {},
+        .pex6 = std::vector{ tr_pex{ addresses[0] }, tr_pex{ addresses[1] }, tr_pex{ addresses[2] } },
+        .errmsg = {},
+        .warning = {},
+        .tracker_id = {}, // not specified in UDP announce
+        .external_ip = {},
+    };
 
     // build the announcer
     auto mediator = MockMediator{};
diff --git a/tests/libtransmission/handshake-test.cc b/tests/libtransmission/handshake-test.cc
index 8c9e60ce8..943230081 100644
--- a/tests/libtransmission/handshake-test.cc
+++ b/tests/libtransmission/handshake-test.cc
@@ -147,16 +147,20 @@ public:
     static auto constexpr PlaintextProtocolName = "\023BitTorrent protocol"sv;
 
     tr_socket_address const DefaultPeerSockAddr{ *tr_address::from_string("127.0.0.1"sv), tr_port::from_host(8080) };
-    tr_handshake::Mediator::TorrentInfo const TorrentWeAreSeeding{ tr_sha1::digest("abcde"sv),
-                                                                   tr_peerIdInit(),
-                                                                   tr_torrent_id_t{ 100 },
-                                                                   true /*is_done*/,
-                                                                   true /*is_running*/ };
-    tr_handshake::Mediator::TorrentInfo const UbuntuTorrent{ *tr_sha1_from_string("2c6b6858d61da9543d4231a71db4b1c9264b0685"sv),
-                                                             tr_peerIdInit(),
-                                                             tr_torrent_id_t{ 101 },
-                                                             false /*is_done*/,
-                                                             true /*is_running*/ };
+    tr_handshake::Mediator::TorrentInfo const TorrentWeAreSeeding{
+        .info_hash = tr_sha1::digest("abcde"sv),
+        .client_peer_id = tr_peerIdInit(),
+        .id = tr_torrent_id_t{ 100 },
+        .is_done = true,
+        .is_running = true,
+    };
+    tr_handshake::Mediator::TorrentInfo const UbuntuTorrent{
+        .info_hash = *tr_sha1_from_string("2c6b6858d61da9543d4231a71db4b1c9264b0685"sv),
+        .client_peer_id = tr_peerIdInit(),
+        .id = tr_torrent_id_t{ 101 },
+        .is_done = false,
+        .is_running = true,
+    };
 
     auto createIncomingIo(tr_session* session)
     {
diff --git a/tests/libtransmission/variant-test.cc b/tests/libtransmission/variant-test.cc
index 329e8c503..c7ae1b43a 100644
--- a/tests/libtransmission/variant-test.cc
+++ b/tests/libtransmission/variant-test.cc
@@ -735,6 +735,7 @@ TEST_F(VariantTest, visitsNodesDepthFirst)
         node.visit(
             [&](auto const& val)
             {
+                // TODO(c++20): use std::remove_cvref_t (P0550R2) when GCC >= 9.1
                 using ValueType = std::decay_t<decltype(val)>;
 
                 if constexpr (