[Bug c++/93317] New: return type deduction fails for templated unary function

[max.kanold@nu-cost.com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93317

            Bug ID: 93317
           Summary: return type deduction fails for templated unary
                    function
           Product: gcc
           Version: 9.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: max.kanold@nu-cost.com
  Target Milestone: ---

The following legal code will not compile as template deduction failed, see
https://godbolt.org/z/AHUHpF

================================
template<typename T>
auto func(const T& t)
//    -> T //mentioning the return type works again
{ return t; }

template<typename Func>
void apply(Func f) {}

int main() {
    apply(func<int>);
}
================================

As stated, there is a workaround, namely mentioning the return type (only if
possible, of course).