[Bug c++/126481] New: [17 regression] Q t's qplugin.h rejected (default member initia lizer for '...’ required before the end of its enclosing class)

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126481

            Bug ID: 126481
           Summary: [17 regression] Qt's qplugin.h rejected (default
                    member initializer for '...’ required before the end
                    of its enclosing class)
           Product: gcc
           Version: 16.0
            Status: UNCONFIRMED
          Keywords: rejects-valid
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: sjames at gcc dot gnu.org
  Target Milestone: ---

```
struct QPluginMetaData
{
        static constexpr int CurrentMetaDataVersion = 1;

        struct Header {
                int version = CurrentMetaDataVersion;
        };

        struct MagicHeader {
                Header header = {};
        };
};
```

```
$ g++ b.cxx
b.cxx:15:34: error: default member initializer for
‘QPluginMetaData::Header::version’ required before the end of its enclosing
class
   15 |                 Header header = {};
      |                                  ^
b.cxx:11:29: note: defined here
   11 |                 int version = CurrentMetaDataVersion;
      |                             ^~~~~~~~~~~~~~~~~~~~~~~~~
```

It's accepted by GCC 16 and Clang.

```

Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/usr/libexec/gcc/x86_64-pc-linux-gnu/17/lto-wrapper
OFFLOAD_TARGET_NAMES=nvptx-none
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-pc-linux-gnu
Configured with:
/var/tmp/portage.notmp/portage/sys-devel/gcc-17.0.9999/work/gcc-17.0.9999/configure
--host=x86_64-pc-linux-gnu --build=x86_64-pc-linux-gnu --prefix=/usr
--bindir=/usr/x86_64-pc-linux-gnu/gcc-bin/17
--includedir=/usr/lib/gcc/x86_64-pc-linux-gnu/17/include
--datadir=/usr/share/gcc-data/x86_64-pc-linux-gnu/17
--mandir=/usr/share/gcc-data/x86_64-pc-linux-gnu/17/man
--infodir=/usr/share/gcc-data/x86_64-pc-linux-gnu/17/info
--with-gxx-include-dir=/usr/lib/gcc/x86_64-pc-linux-gnu/17/include/g++-v17
--disable-silent-rules --disable-dependency-tracking
--with-python-dir=/share/gcc-data/x86_64-pc-linux-gnu/17/python
--enable-libphobos --enable-objc-gc
--enable-languages=c,c++,d,objc,obj-c++,fortran,ada,algol68,m2,jit
--enable-obsolete --enable-secureplt --disable-werror --with-system-zlib
--enable-nls --without-included-gettext --disable-libunwind-exceptions
--enable-checking=yes,extra,rtl --with-bugurl=https://bugs.gentoo.org/
--with-pkgversion='Gentoo Hardened 17.0.9999 p, commit
2ca94d918af8824540e40bab413e80edb58246a6' --with-gcc-major-version-only
--enable-libstdcxx-time --enable-lto --disable-libstdcxx-pch
--enable-linker-build-id --enable-libada --enable-shared --enable-threads=posix
--enable-__cxa_atexit --enable-clocale=gnu --with-tls=gnu2 --enable-multilib
--with-multilib-list=m32,mx32,m64 --disable-fixed-point --with-abi=m64
--enable-targets=all --enable-offload-defaulted
--enable-offload-targets=nvptx-none --enable-libgomp --disable-libssp
--disable-cet --enable-systemtap --enable-valgrind-annotations
--enable-valgrind-interop --disable-vtable-verify --disable-libvtv --with-zstd
--with-isl --disable-isl-version-check --enable-default-pie --enable-host-pie
--enable-host-bind-now --enable-default-ssp --disable-fixincludes
--with-gxx-libcxx-include-dir=/usr/include/c++/v1 --enable-host-shared
--enable-libgdiagnostics --with-build-config='bootstrap-O3 bootstrap-lto-lean'
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 17.0.0 20260728 (experimental)
0531c407b983f322ade13feef6966b3593b8b989 (Gentoo Hardened 17.0.9999 p, commit
2ca94d918af8824540e40bab413e80edb58246a6)
```