Incompatibility with C++20

[fmorgner]

Actual behavior

Compilation fails when using the jpeg io extension (and others).

In file included from /home/sophia/.conan/data/boost/1.76.0/_/_/package/dc8aedd23a0f0a773a5fcdcfe1ae3e89c4205978/include/boost/gil/extension/io/jpeg/read.hpp:13,
                 from /home/sophia/.conan/data/boost/1.76.0/_/_/package/dc8aedd23a0f0a773a5fcdcfe1ae3e89c4205978/include/boost/gil/extension/io/jpeg.hpp:11,
                 from /home/sophia/Projects/system/wanda/src/wanda/wallpaper.cpp:8:
/home/sophia/.conan/data/boost/1.76.0/_/_/package/dc8aedd23a0f0a773a5fcdcfe1ae3e89c4205978/include/boost/gil/extension/io/jpeg/tags.hpp:144:35: error: expected unqualified-id before ‘)’ token
  144 |     image_read_settings<jpeg_tag>()
      |                                   ^
make[2]: *** [CMakeFiles/wandad.dir/build.make:118: CMakeFiles/wandad.dir/src/wanda/wallpaper.cpp.o] Error 1
make[1]: *** [CMakeFiles/Makefile2:170: CMakeFiles/wandad.dir/all] Error 2
make: *** [Makefile:146: all] Error 2

Expected behavior

Compilation should succeed.

C++ Minimal Working Example

#include <boost/gil/extension/io/jpeg.hpp>
int main() {}

Environment

With the release of ISO14882:2020, simple-template-ids are no longer considered valid as the id-expression of the ptr-declarator when declaring the constructor of a class template specialization.

See also:

• DR 2237 @ http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html
• https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100997
• https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97202
• https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97202

• Compiler version: GCC 11.1
• Build settings: -std=c++20
• Version (Git ref or <boost/version.hpp>): 1.76.0