[Bug c++/114933] New: [15 Regression] mcfgthread-1.6.1 typecheck failure: error: explicit specializations are not permitted here

["slyfox at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114933

            Bug ID: 114933
           Summary: [15 Regression] mcfgthread-1.6.1 typecheck failure:
                    error: explicit specializations are not permitted here
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: slyfox at gcc dot gnu.org
  Target Milestone: ---

mcfgthread-1.6.1 build failure started happening a few days ago on `gcc`
`master`. On r15-116-gff4dc8b10a421c minimized example failure looks this way:

    // $ cat a.cc
    extern "C++" template<bool> struct __MCF_static_assert;
    extern "C++" template<> struct __MCF_static_assert<true> { };

Fails as:

$ g++ -c a.cc -std=c++20
a.cc:2:14: error: explicit specializations are not permitted here
    2 | extern "C++" template<> struct __MCF_static_assert<true> { };
      |              ^~~~~~~~~~

For comparison 13.2.0 builds the example successfully:

$ g++-13.2.0 -c a.cc -std=c++20
<ok>

Compiler details:

$ g++ -v |& unnix
Using built-in specs.
COLLECT_GCC=/<<NIX>>/gcc-15.0.0/bin/g++
COLLECT_LTO_WRAPPER=/<<NIX>>/gcc-15.0.0/libexec/gcc/x86_64-unknown-linux-gnu/15.0.0/lto-wrapper
Target: x86_64-unknown-linux-gnu
Configured with: ../source/configure --prefix=/<<NIX>>/gcc-15.0.0
--with-gmp-include=/<<NIX>>/gmp-6.3.0-dev/include
--with-gmp-lib=/<<NIX>>/gmp-6.3.0/lib
--with-mpfr-include=/<<NIX>>/mpfr-4.2.1-dev/include
--with-mpfr-lib=/<<NIX>>/mpfr-4.2.1/lib --with-mpc=/<<NIX>>/libmpc-1.3.1
--with-native-system-header-dir=/<<NIX>>/glibc-2.39-31-dev/include
--with-build-sysroot=/
--with-gxx-include-dir=/<<NIX>>/gcc-15.0.0/include/c++/15.0.0/
--program-prefix= --enable-lto --disable-libstdcxx-pch
--without-included-gettext --with-system-zlib --enable-checking=release
--enable-static --enable-languages=c,c++ --disable-multilib --enable-plugin
--disable-libcc1 --with-isl=/<<NIX>>/isl-0.20 --disable-bootstrap
--build=x86_64-unknown-linux-gnu --host=x86_64-unknown-linux-gnu
--target=x86_64-unknown-linux-gnu
Thread model: posix
Supported LTO compression algorithms: zlib
gcc version 15.0.0 99999999 (experimental) (GCC)