[LLVM] GCC15 prints thousands of warnings when compiling.

[carlo-bramini]

I tried to build the latest development sources with GCC-15.1, the build process runs but I noticed that thousands of warnings like this one are printed on the console:

cc1plus: note: unrecognized command-line option ‘-Wno-unnecessary-virtual-specifier’ may have been intended to silence earlier diagnostics

Actually, the effect is just a warning message, but perhaps it would be worth to avoid it, if it's possible.
This flags seems to be added only here:

llvm-project/llvm/cmake/modules/HandleLLVMOptions.cmake

Line 888 in 926c201

add_flag_if_supported("-Wno-unnecessary-virtual-specifier" CXX_SUPPORTS_UNNECESSARY_VIRTUAL_FLAG)

by using a macro called add_flag_if_supported() into the code.
Unfurtunately, it seems that it doesn't detect that GCC is not understanding this option.

[Alcaro]

GCC is supposed to ignore unknown -Wno-x flags if there are no other diagnostics.

$ echo '' | gcc -Wno-walrus -xc - -o/dev/null -c
$ echo 'int e() { return 1/0; }' | gcc -Wno-walrus -xc - -o/dev/null -c
<stdin>: In function ‘e’:
<stdin>:1:19: warning: division by zero [-Wdiv-by-zero]
At top level:
cc1: note: unrecognized command-line option ‘-Wno-walrus’ may have been intended to silence earlier diagnostics

Either it's not ignoring the -Wno-x as it should, or there's a different diagnostic nearby. If former, that's a GCC bug; if latter, that's its own issue. In both cases, I'm not sure if that's a bug in the cmakefiles.