Fix CI on develop after #5600, #5607, and #5755

[nlohmann]

On develop (23d3b37), the Ubuntu and Publish documentation workflows and the AppVeyor build fail. There are five separate causes. Most of the other red Ubuntu jobs were only cancelled by fail-fast.

1. -Warith-conversion in the CBOR reader (#5607)

ci_test_gcc and ci_test_standards_gcc fail at binary_reader.hpp:668 when number_integer_t is short (the narrow-type tests in unit-msgpack.cpp). static_cast<number_integer_t>(-1) - static_cast<number_integer_t>(number) is promoted to int. Converting it back to short implicitly is a -Werror=arith-conversion error. The fix casts the result back with conditional_static_cast, as the BON8 reader already does.

2. Deleted friend operators with old compilers (#5755)

With JSON_DELETE_DEPRECATED_FUNCTIONS, the deprecated stream operators j >> o and j << i were friend ... = delete; inside basic_json. Older compilers reject this:

• GCC 4.8 and 4.9: can't initialize friend function
• Clang 7, 8 and 9: redefinition of 'operator>>' once two specializations exist

I checked the affected versions with minimal reproducers in the CI Docker images. Clang 3.4/3.5 and Clang ≥ 10 accept it.

The fix keeps the friend definitions only when the functions are not deleted. When they are, deleted function templates at namespace scope (NLOHMANN_BASIC_JSON_TPL_DECLARATION) take their place, and ADL finds them the same way. unit-delete_deprecated_functions.cpp now compiles with Clang 3.4, 7, 8, 9 and latest, and with GCC 4.9.

3. Documentation style check (#5600)

check_structure.py rejects json_use_objects_for_enum_keyed_maps.md: example titles need the "Example: …" form, and the page had no docset entry. Both are fixed. make style_check and mkdocs build --strict pass locally.

4. libstdc++ <format> under -fsanitize=integer (toolchain)

ci_test_clang_sanitizer (silkeh/clang:22, libstdc++ 14.2) fails in test-std-format_cpp20 at format:3906. There, _Scanner(basic_string_view<_CharT>, size_t __nargs = -1) converts -1 to size_t, which implicit-integer-sign-change reports for every std::format call. This is GCC bug 119429, not a library bug; develop fails the same way. Thanks to @fhgffy for pinning it down in #5760.

The new cmake/clang_sanitizer_ignorelist.txt, passed via -fsanitize-ignorelist, excludes only that check and only that header. A deliberate sign change in a test file is still reported, and unit-std-format.cpp passes under the CI's UBSan flags in silkeh/clang:22.

5. AppVeyor: MSVC 2015, 2017 and 2019 (#5607, #5751, #5754)

All AppVeyor jobs fail, and PR builds that merged develop on 10-04 failed the same way:

• C2668 fpclassify (all versions, Handle numbers that do not fit narrow number types in the binary readers #5607): emit_signed/emit_unsigned passed integers to emit_float, which calls std::isfinite. MSVC's <cmath> has no integer overloads for it. Integers are now converted to long double first, as get_cbor_negative_integer already does.
• C3203 (2015/2017, Make scalar comparisons noexcept only when converting the scalar cannot throw #5751): in the noexcept of the in-class friend scalar comparisons, these versions take basic_json as the class template. On Compiler Explorer, MSVC 2019 16.0 rejects member aliases (basic_json_t) and template parameters in the same position. Only the function parameters are visible there, so the friends now use std::is_nothrow_constructible<detail::uncvref_t<decltype(lhs)>, ScalarType>. I checked a reproducer with MSVC 2019 16.0, 16.11 and latest; 2015/2017 are not on Compiler Explorer. The unit-noexcept.cpp C2338 failure is a consequence of this error.
• C4800 (2015, Fix CI on develop after merging the ready-to-merge PRs #5754): MSVC 2015 warns about any conversion to bool, even an explicit static_cast, which /WX turns into an error. The enum conversions for an underlying type bool (Default enum conversion: an enum with underlying type bool can be serialized but not deserialized #5671) now go through the new detail::bool_aware_static_cast, which compares with zero for bool. The test wraps its CHECKs in double parentheses, so doctest does not print the enum via its underlying type.
• Crash in test-allocator (2015 Debug, Converting between basic_json specializations (e.g. json to ordered_json) overflows the stack on deep input #5650's test): the debug STL constructs the containers' debug proxies through the allocator in noexcept constructors, so the test's failing countdown_allocator crashes the program instead of throwing std::bad_alloc. The test case is skipped there, using the guard Create a value before giving it its type #5585 introduced for the same reason.
• Detection checks of deleted functions (2015, tests from Add JSON_DELETE_DEPRECATED_FUNCTIONS to delete the deprecated functions #5755 and contains(0), find(0), and count(0) compile and crash by constructing the key from a null pointer #5657): MSVC 2015 does not treat selecting a deleted function in decltype as a substitution failure. Its detection traits therefore report the deleted overloads as callable, although calling them still fails to compile. Those checks are skipped for _MSC_VER < 1910. The runtime section of unit-delete_deprecated_functions.cpp now also uses os << j and is >> j, so MSVC 2015 still compiles the stream operators with the macro set.
• Timeout (2017): the VS 2017 jobs hit AppVeyor's 60-minute limit per job while still compiling (77 of about 108 test targets after 58 minutes). They pass /std:c++17 for everything anyway, so they now build only the C++17 variant of each test (JSON_TestStandards=17). They are also split into two jobs each with a new test-only CMake option, JSON_TestShard=<index>/<count>, which keeps every count-th unit test file starting at index (48 and 44 of the 92 unit test targets). Locally, the two shards together give exactly the full set of tests. CMAKE_OPTIONS is no longer quoted in appveyor.yml, so that it can hold two options.
• std::terminate with std::optional (2017, Fix CI on develop after merging the ready-to-merge PRs #5754): MSVC 2017 evaluates std::is_nothrow_assignable<json&, const T&> as true even if T's to_json throws. So to_json(json&, const std::optional<T>&) was noexcept there, and the exception from to_json(std::optional<T>) is noexcept: an exception from the contained value calls std::terminate #5642's test called std::terminate instead of propagating (C2607 in the static_assert showed the same thing at compile time). On MSVC 2017, that conversion is now never noexcept; all other compilers keep the exact condition. The static_asserts on the condition are skipped for MSVC 2017; the runtime check that the exception propagates still runs there. This is a real library fix for MSVC 2017 users, not just a test fix.

Validation

• A full ci_test_gcc build in gcc:latest with the CI flags (-Werror) compiles every target. The only failing tests are FetchContent/git-clone tests, which need network access that my container did not have.
• Amalgamation was regenerated with make amalgamate.

Public API

No breaking changes. The noexcept of the scalar comparison operators is unchanged; it is only spelled differently. Without JSON_DELETE_DEPRECATED_FUNCTIONS, nothing changes. With it, the two deprecated stream operators are still deleted. They are now namespace-scope function templates instead of hidden friends, so using them is still a compile error.

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