[Bug c++/100739] New: Definition of function template doesn't match declaration

[raffael at casagrande dot ch]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100739

            Bug ID: 100739
           Summary: Definition of function template doesn't match
                    declaration
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: raffael at casagrande dot ch
  Target Milestone: ---

The following code fails to compile with the newest gcc (gcc 12.0.0 20210523):
-----------------------------
#include <concepts>
#include <array>

template <class T>
requires std::integral<T> using debug_t = T;

template<std::integral T>
struct X {
  auto operator()(std::integral auto const& i) const ->
std::array<debug_t<decltype(i)>,1>;
};

template<std::integral T>
auto X<T>::operator()(std::integral auto const& i) const ->
std::array<debug_t<decltype(i)>,1>{

};
--------------------------
The error message is:
--------------------------
<source>:13:6: error: no declaration matches 'std::array<debug_t<decltype (i)>,
1> X<T>::operator()(const auto:4&) const'
   13 | auto X<T>::operator()(std::integral auto const& i) const ->
std::array<debug_t<decltype(i)>,1>{
      |      ^~~~
<source>:9:8: note: candidate is: 'template<class T> template<class auto:3> 
requires  integral<auto:3> std::array<debug_t<decltype (i)>, 1>
X<T>::operator()(const auto:3&) const'
    9 |   auto operator()(std::integral auto const& i) const ->
std::array<debug_t<decltype(i)>,1>;
      |        ^~~~~~~~
<source>:8:8: note: 'struct X<T>' defined here
    8 | struct X {
      |        ^
--------------------------

If we further simplify the code in one of the following ways, the problem
disappears:
1) If we remove the `std::array<...>`
2) If we remove the requires clause in the definition of debug_t