[r16-5727 Regression] FAIL: g++.dg/modules/omp-4_a.C -std=c++26 (test for excess errors) on Linux/x86_64

[Haochen Jiang]

On Linux/x86_64,

858f300727833740f74d90378a0f302d4254ef37 is the first bad commit
commit 858f300727833740f74d90378a0f302d4254ef37
Author: Nathaniel Shead <nathanieloshead@gmail.com>
Date:   Sat Nov 15 15:27:13 2025 +1100

    c++: Support template block-scope OpenMP user-defined reductions in modules [PR119864]

caused

FAIL: g++.dg/modules/omp-4_a.C module-cmi p1 (gcm.cache/p1.gcm)
FAIL: g++.dg/modules/omp-4_a.C -std=c++17 (internal compiler error: in write_function_def, at cp/module.cc:12937)
FAIL: g++.dg/modules/omp-4_a.C -std=c++17 (test for excess errors)
FAIL: g++.dg/modules/omp-4_a.C -std=c++20 (internal compiler error: in write_function_def, at cp/module.cc:12937)
FAIL: g++.dg/modules/omp-4_a.C -std=c++20 (test for excess errors)
FAIL: g++.dg/modules/omp-4_a.C -std=c++26 (internal compiler error: in write_function_def, at cp/module.cc:12937)
FAIL: g++.dg/modules/omp-4_a.C -std=c++26 (test for excess errors)

with GCC configured with

../../gcc/configure --prefix=/export/users3/haochenj/src/gcc-bisect/master/master/r16-5727/usr --enable-clocale=gnu --with-system-zlib --with-demangler-in-ld --with-fpmath=sse --enable-languages=c,c++,fortran --enable-cet --without-isl --enable-libmpx x86_64-linux --disable-bootstrap

To reproduce:

$ cd {build_dir}/gcc && make check RUNTESTFLAGS="modules.exp=g++.dg/modules/omp-4_a.C --target_board='unix{-m32}'"
$ cd {build_dir}/gcc && make check RUNTESTFLAGS="modules.exp=g++.dg/modules/omp-4_a.C --target_board='unix{-m32\ -march=cascadelake}'"
$ cd {build_dir}/gcc && make check RUNTESTFLAGS="modules.exp=g++.dg/modules/omp-4_a.C --target_board='unix{-m64}'"
$ cd {build_dir}/gcc && make check RUNTESTFLAGS="modules.exp=g++.dg/modules/omp-4_a.C --target_board='unix{-m64\ -march=cascadelake}'"

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(If you met problems with cascadelake related, disabling AVX512F in command line might save that.)
(However, please make sure that there is no potential problems with AVX512.)