clang-cl + `Debug`: `/WX` promotes three tolerated diagnostics to hard errors — the build cannot complete, starting with `/experimental:c11atomics` being probed as supported

[AndreaLanfranchi]

A standalone Debug build with clang-cl never finishes compiling. CMake sets MSVC for clang-cl
as well as for cl, so libmdbx adds /WX and applies MSVC-only flags that clang-cl merely warns
about — and /WX turns each warning into an error.

Environment

libmdbx	v0.14.3 (f7a3a932)
compiler	clang-cl 22.1.3, target x86_64-pc-windows-msvc
CMake	4.3.1-msvc1, Ninja generator
OS	Windows 11 26200, x86_64

Reproduce

cmake -S libmdbx -B build -G Ninja -DCMAKE_BUILD_TYPE=Debug \
  -DCMAKE_C_COMPILER=clang-cl -DCMAKE_CXX_COMPILER=clang-cl -DMDBX_BUILD_TOOLS=ON
cmake --build build

The three diagnostics

1. /experimental:c11atomics, every C and C++ TU — this one alone stops the build:

clang-cl: error: argument unused during compilation: '/experimental:c11atomics'
          [-Werror,-Wunused-command-line-argument]

Chain: cmake/compiler.cmake:436 runs
check_c_compiler_flag("/experimental:c11atomics" MSVC_C11_MAD_ATOMICS). clang-cl does not implement
the flag but only warns, so the probe passes and the cache records
MSVC_C11_MAD_ATOMICS:INTERNAL=1. cmake/compiler.cmake:913 then adds /WX, and
CMakeLists.txt:775 applies the flag on the strength of the cached probe.

2. mdbx.h++:1499 — present in both static and shared builds:

mdbx.h++(1499,7): error: attribute 'unlikely' has no effect when annotating
                  an 'if constexpr' statement [-Werror,-Wignored-attributes]

MDBX_CXX20_UNLIKELY is applied to the branch of an if MDBX_IF_CONSTEXPR (...) in
propagate().

3. dllexport on explicit instantiations — with MDBX_BUILD_SHARED_LIBRARY=ON (the default):

mdbx.h++(5067,1): error: explicit instantiation declaration should not be 'dllexport'
                  [-Werror,-Wdllexport-explicit-instantiation-decl]
mdbx.h++(5070,1): error: same
mdbx.c++(2197,1): error: explicit instantiation definition is not exported without 'dllexport'
                  [-Werror,-Wignored-attributes]
mdbx.c++(2200,1): error: same

These disappear under -DMDBX_BUILD_SHARED_LIBRARY=OFF; (1) and (2) remain.

Why Release and subdirectory consumers do not see it

/WX is added only for Debug, or under CI, or with a multi-config generator
(cmake/compiler.cmake:907-913), so Release merely warns.

The /WX block also lives inside macro(setup_compile_flags) (cmake/compiler.cmake:752), so a
project that adds libmdbx via add_subdirectory() without calling that macro is unaffected. The
flags are still applied to the targets in that case — we see /experimental:c11atomics on every
libmdbx compile line — so the warnings are emitted on every build; they simply are not fatal.

Suggested fix

For (1), either probe under the conditions the flag is later used in:

if(MSVC)
  set(CMAKE_REQUIRED_FLAGS "/WX")
  check_c_compiler_flag("/experimental:c11atomics" MSVC_C11_MAD_ATOMICS)
  unset(CMAKE_REQUIRED_FLAGS)
endif()

or skip the flag for clang-cl, which does not implement it:

if(MSVC AND NOT CMAKE_COMPILER_IS_CLANG)
  check_c_compiler_flag("/experimental:c11atomics" MSVC_C11_MAD_ATOMICS)
endif()

The second matches the existing guard at cmake/compiler.cmake:524.

(2) and (3) are source-level and independent of the flag handling.

Workaround

Build a non-Debug configuration. -DMSVC_C11_MAD_ATOMICS=0 clears (1) on its own but leaves (2)
and (3), so it is not sufficient by itself.

[erthink]

I'll try to fix it, but...

It's a very strange broken compiler.
I don't understand what Microsoft is doing – they took clang and broke it by sewing some falling parts of MSVC there.

It is not compatible with either CLANG or MSVC, but according to predefined macros it corresponds to both ones.
Besides above, it crashes by ICE (Internal Compiler Error) or "access violation" 10 times more than the previous "champion" MSVC.
etc.