After 3427e31331677ca826c5588c87924214f7e5c54b, "clang -std=c++17" (using libstdc++ headers), on a file that has simply "#include <variant>" fails with this error: In file included from <source>:1: /opt/compiler-explorer/gcc-snapshot/lib/gcc/x86_64-linux-gnu/11.0.0/../../../../include/c++/11.0.0/variant:1032:10: error: no matching constructor for initialization of 'std::__nonesuch' return __nonesuch{}; ^ ~~ /opt/compiler-explorer/gcc-snapshot/lib/gcc/x86_64-linux-gnu/11.0.0/../../../../include/c++/11.0.0/type_traits:2953:5: note: candidate constructor not viable: requires 1 argument, but 0 were provided __nonesuch(__nonesuch const&) = delete; ^ (https://godbolt.org/z/951jYr) I believe Clang is correct to reject this code, due to http://eel.is/c++draft/temp.res#general-8.1 """ The program is ill-formed, no diagnostic required, if: - no valid specialization can be generated for a template or a substatement of a constexpr if statement within a template and the template is not instantiated """ The invalid return statement probably needs to be made dependent on __visit_ret_type_mismatch just like the static_assert.
Mine. I have a patch for it already, doing testing and submission chores...
Patch available: https://gcc.gnu.org/pipermail/gcc-patches/2020-October/556323.html
The master branch has been updated by Ville Voutilainen <ville@gcc.gnu.org>: https://gcc.gnu.org/g:1f65bf2aa65609c0cd88af1b83191d37d6729f46 commit r11-4024-g1f65bf2aa65609c0cd88af1b83191d37d6729f46 Author: Ville Voutilainen <ville.voutilainen@gmail.com> Date: Sat Oct 17 22:08:50 2020 +0300 libstdc++: Fix visitor return type diagnostics [PR97449] libstdc++-v3/ChangeLog: PR libstdc++/97449 * include/std/variant (__gen_vtable_impl<>::_S_apply_single_alt): Diagnose visitor return type mismatches here.. (__gen_vtable_impl</*base case*/>::_S_apply): ..not here.
Fixed.