[clang] [Clang] [Sema] Error on reference types inside a union with msvc 1900+ (PR #102851)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

I think those are failing because of unexpected passes in libc++ tests:
```
******************** TEST 'llvm-libc++-static-clangcl.cfg.in :: libcxx/utilities/expected/expected.void/transform_error.mandates.verify.cpp' FAILED ********************
Exit Code: 0

Command Output (stdout):
--
# COMPILED WITH
C:/b/s/w/ir/x/w/llvm_build/./bin/clang-cl.exe C:\b\s\w\ir\x\w\llvm-llvm-project\libcxx\test\libcxx\utilities\expected\expected.void\transform_error.mandates.verify.cpp --driver-mode=g++ --target=x86_64-pc-windows-msvc -fms-runtime-lib=static -nostdinc++ -I C:/b/s/w/ir/x/w/llvm_build/include/x86_64-pc-windows-msvc19.34.31933/c++/v1 -I C:/b/s/w/ir/x/w/llvm_build/include/c++/v1 -I C:/b/s/w/ir/x/w/llvm-llvm-project/libcxx/test/support -D_CRT_SECURE_NO_WARNINGS -D_CRT_NONSTDC_NO_WARNINGS -D_CRT_STDIO_ISO_WIDE_SPECIFIERS -DNOMINMAX -std=c++26 -Werror -Wall -Wctad-maybe-unsupported -Wextra -Wshadow -Wundef -Wunused-template -Wno-unused-command-line-argument -Wno-attributes -Wno-pessimizing-move -Wno-noexcept-type -Wno-atomic-alignment -Wno-reserved-module-identifier -Wdeprecated-copy -Wdeprecated-copy-dtor -Wno-user-defined-literals -Wno-tautological-compare -Wsign-compare -Wunused-variable -Wunused-parameter -Wunreachable-code -Wno-unused-local-typedef -Wno-local-type-template-args -Wno-c++11-extensions -Wno-unknown-pragmas -Wno-pass-failed -Wno-mismatched-new-delete -Wno-redundant-move -Wno-self-move -D_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER -D_LIBCPP_ENABLE_EXPERIMENTAL -D_LIBCPP_HARDENING_MODE=_LIBCPP_HARDENING_MODE_NONE -Werror=thread-safety -Wuser-defined-warnings  -fsyntax-only -Wno-error -Xclang -verify -Xclang -verify-ignore-unexpected=note -ferror-limit=0
# executed command: C:/b/s/w/ir/x/w/llvm_build/./bin/clang-cl.exe 'C:\b\s\w\ir\x\w\llvm-llvm-project\libcxx\test\libcxx\utilities\expected\expected.void\transform_error.mandates.verify.cpp' --driver-mode=g++ --target=x86_64-pc-windows-msvc -fms-runtime-lib=static -nostdinc++ -I C:/b/s/w/ir/x/w/llvm_build/include/x86_64-pc-windows-msvc19.34.31933/c++/v1 -I C:/b/s/w/ir/x/w/llvm_build/include/c++/v1 -I C:/b/s/w/ir/x/w/llvm-llvm-project/libcxx/test/support -D_CRT_SECURE_NO_WARNINGS -D_CRT_NONSTDC_NO_WARNINGS -D_CRT_STDIO_ISO_WIDE_SPECIFIERS -DNOMINMAX -std=c++26 -Werror -Wall -Wctad-maybe-unsupported -Wextra -Wshadow -Wundef -Wunused-template -Wno-unused-command-line-argument -Wno-attributes -Wno-pessimizing-move -Wno-noexcept-type -Wno-atomic-alignment -Wno-reserved-module-identifier -Wdeprecated-copy -Wdeprecated-copy-dtor -Wno-user-defined-literals -Wno-tautological-compare -Wsign-compare -Wunused-variable -Wunused-parameter -Wunreachable-code -Wno-unused-local-typedef -Wno-local-type-template-args -Wno-c++11-extensions -Wno-unknown-pragmas -Wno-pass-failed -Wno-mismatched-new-delete -Wno-redundant-move -Wno-self-move -D_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER -D_LIBCPP_ENABLE_EXPERIMENTAL -D_LIBCPP_HARDENING_MODE=_LIBCPP_HARDENING_MODE_NONE -Werror=thread-safety -Wuser-defined-warnings -fsyntax-only -Wno-error -Xclang -verify -Xclang -verify-ignore-unexpected=note -ferror-limit=0

--

********************
```

CC @ldionne @philnik777 @mordante

https://github.com/llvm/llvm-project/pull/102851