[Bug c/105177] New: GCC should warn if pragma redefine_extname fails due to missing declaration

[timschumi at gmx dot de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105177

            Bug ID: 105177
           Summary: GCC should warn if pragma redefine_extname fails due
                    to missing declaration
           Product: gcc
           Version: 11.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: timschumi at gmx dot de
  Target Milestone: ---

The file `atomic-repro.c` compiles without warnings or errors when compiled
using the following compiler invocation:


$ cat atomic-repro.c
#pragma redefine_extname __atomic_load_c __atomic_load

void __atomic_load_c(void)
{
    return;
}

$ gcc -Wall -Wextra -c atomic-repro.c


However, the resulting assembly/binary ends up using the original symbol name
instead of the new one.

I realize that the symbol using the old name may very well be technically
correct behavior (presumably due to the "linkage" restriction), but it feels
like this specific case at least could benefit from a warning to prevent people
from running into issues unexpectedly.

GCC has been configured with the following:

$ gcc -v
Using built-in specs.
COLLECT_GCC=/usr/bin/gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-pc-linux-gnu/11.2.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: /build/gcc/src/gcc/configure
--enable-languages=c,c++,ada,fortran,go,lto,objc,obj-c++,d --enable-bootstrap
--prefix=/usr --libdir=/usr/lib --libexecdir=/usr/lib --mandir=/usr/share/man
--infodir=/usr/share/info --with-bugurl=https://bugs.archlinux.org/
--with-linker-hash-style=gnu --with-system-zlib --enable-__cxa_atexit
--enable-cet=auto --enable-checking=release --enable-clocale=gnu
--enable-default-pie --enable-default-ssp --enable-gnu-indirect-function
--enable-gnu-unique-object --enable-linker-build-id --enable-lto
--enable-multilib --enable-plugin --enable-shared --enable-threads=posix
--disable-libssp --disable-libstdcxx-pch --disable-werror
--with-build-config=bootstrap-lto --enable-link-serialization=1
gdc_include_dir=/usr/include/dlang/gdc
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 11.2.0 (GCC)