[Bug c++/109001] New: “no declaration matches” for complicated non-type template parameters

["headch at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109001

            Bug ID: 109001
           Summary: “no declaration matches” for complicated non-type
                    template parameters
           Product: gcc
           Version: 12.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: headch at gmail dot com
  Target Milestone: ---

I’m reasonably certain (but not completely) that this is valid code:

#include <algorithm>
#include <cstddef>

template<size_t x>
class C {
public:
template<size_t y>
C<std::max(x, y)> set();
};

template<size_t x>
template<size_t y>
C<std::max(x, y)> C<x>::set() {
return {};
}

however, GCC rejects it with “error: no declaration matches ‘C<std::max<long
unsigned int>(x, y)> C<x>::set()’” on the out-of-body function definition. This
is accepted in GCC 11.3, as well as the latest versions of Clang and MSVC on
gcc.godbolt.org. Interestingly, GCC 12 also accepts the code if I change the
function definition (just the definition, not the in-body declaration) to use
trailing-return-type syntax:

#include <algorithm>
#include <cstddef>

template<size_t x>
class C {
public:
template<size_t y>
C<std::max(x, y)> set();
};

template<size_t x>
template<size_t y>
auto C<x>::set() -> C<std::max(x, y)> {
return {};
}