Fix CI on develop after merging the ready-to-merge PRs

[nlohmann]

Several PRs were merged into develop on 2026-10-04 with red or cancelled CI. Each was fine on its own, but together, and in configurations CI never got to run (fail-fast cancelled most matrices), they broke develop. This PR fixes everything that turned up.

How this was verified

The CI targets were reproduced locally with keep-going builds (ninja -k 0), so all errors surface at once rather than only the first one per job:

Check	Environment	Result
ci_test_gcc	gcc:latest (GCC 16.2)	✅
ci_test_clang, ci_clang_tidy, ci_single_binaries	silkeh/clang:22 + clang-tools, iwyu	✅
ci_cmake_options: noglobaludls, noexceptions, noimplicitconversions, diagnostics, diagnostic_positions, legacycomparison, disableenumserialization, disabletuplereferenceconversion, strict_nul_handling, no_thread_local	Apple clang, C++11 and C++20	✅ all 10
Unit tests touched by the merged PRs	clang 3.5, GCC 4.8, GCC 5 (syntax only)	✅
make check-amalgamation, docs style_check	host	✅

The FetchContent tests fail inside a Docker container built from a git worktree; they are unrelated and pass in CI. MSVC and MinGW can't run locally, so those were checked statically (C4127/C2512 patterns, object sizes).

Changes

Library

• unit-alt-string did not compile (Make cross-string-type basic_json conversion explicit without implicit conversions #5591 × the iterative conversion): when the keys cannot be converted, the conversion past the nesting limit now leaves the value to the serializers (Conversion from alt_json to json produces incorrect result #3425).
• binary_reader: the constructor parameter error_handler shadowed the member (-Wshadow; Add an error_handler parameter for UTF-8 to the binary readers and writers #5746).
• basic_json(copy_construct_tag, …) is noexcept when copying the base class is (GCC 16 -Wnoexcept; Copy-construct the base class of a deep copy's elements, not assign it #5690).
• Legacy discarded-value comparisons with the scalar on the left are noexcept only when converting the scalar is (Add scalar-on-left overloads for legacy discarded comparisons in C++20 #5682 × Make scalar comparisons noexcept only when converting the scalar cannot throw #5751).
• compare_leaves uses std::is_eq/is_lt/is_gt (-Wzero-as-null-pointer-constant; Classify leaves with operator<=> itself past the nesting bound #5686).
• serializer: MSVC C4127 on the EnsureAscii template parameter is silenced the same way as elsewhere (Follow each binary format's UTF-8 rule: strict writers (CBOR/UBJSON/BJData/BSON), lenient readers #5741/Add an error_handler parameter for UTF-8 to the binary readers and writers #5746).
• clang-tidy: binary_writer's sanitized reference, ordered_map::find_impl takes a const reference (Deduplicate basic_json internals; make insert(pos, json&&) move #5727), the parse_array_index switches (Remove dead metaprogramming and duplicated code in traits and pointers #5728); IWYU: ordered_map keeps <memory>.

Tests

• unit-conversions.cpp is split in two, so MinGW clang 18 can link it ("relocation truncated to fit").
• unit-wstring: the 16-bit wchar_t expectations since Reject malformed UTF-16/UTF-32 units in wide-string input #5704.
• Discarded results in CHECK_THROWS (GCC -Werror=unused-result), huge_string_t default constructor (MSVC, GCC 5, clang 3.5), literals with JSON_GlobalUDLs=OFF (Fix value(json_pointer, default) aborting under JSON_NOEXCEPTION #5700), the JSON_DIAGNOSTICS prefix (Follow each binary format's UTF-8 rule: strict writers (CBOR/UBJSON/BJData/BSON), lenient readers #5741), exception-dependent tests under JSON_NOEXCEPTION (Fix update() and merge_patch() when the argument is *this or one of its members #5678, Remove test debt: dead guards, discarded results, and unreferenced files #5732), and clang-tidy/-Werror clean-ups.

CI and generated files

• ci_cmake_flags: install libidn11, which the CMake 3.5.0 release binary needs; Fix CI jobs that check less than they claim; move arm64 to GitHub #5733 moved the job to ubuntu:24.04, which no longer ships it.
• ci_single_binaries: json_literals.hpp (Move the user-defined string literals to <nlohmann/json_literals.hpp> and add JSON_NO_AUTOMATIC_UDLS #5610) is reported but not gated, like json.hpp, since IWYU does not follow their intended include cycle.
• BUILD.bazel and nlohmann_json.natvis are regenerated (Add an error_handler parameter for UTF-8 to the binary readers and writers #5746's new header, Follow each binary format's UTF-8 rule: strict writers (CBOR/UBJSON/BJData/BSON), lenient readers #5741's ABI tag).
• Docs: example titles and the docset entry required by the style check (Follow each binary format's UTF-8 rule: strict writers (CBOR/UBJSON/BJData/BSON), lenient readers #5741, Make cross-string-type basic_json conversion explicit without implicit conversions #5591).

Public API

No breaking changes. Two noexcept specifications become more precise: basic_json(copy_construct_tag, …) gains a conditional noexcept, and the legacy scalar-on-left <=/>= lose an unconditional one where converting the scalar can throw. The latter only applies with JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON in C++20, and only matters to code that queried noexcept on these operators.

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