[Bug c++/66923] New: Variable template initialized using auto deduces wrong type

[sneves at dei dot uc.pt]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66923

            Bug ID: 66923
           Summary: Variable template initialized using auto deduces wrong
                    type
           Product: gcc
           Version: 5.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: sneves at dei dot uc.pt
  Target Milestone: ---

Consider the following sample:

    template<int> struct S {};

    template<int N>
    constexpr auto V = S<N>{};
    // constexpr S<N> V{}; // This works

    template<typename T>
    auto f(T) {
      return V<(
    #if defined(TRIGGER_ERROR)
          (void)T(),
    #endif
      0)>;
    }

    int main() {
      f(0);
    }

If TRIGGER_ERROR is defined, the definition of V is dependent on the deduced
type T. In this case, GCC will seemingly deduce that the type of V<int> is T:

    error: cannot convert 'S<0>' to 'const int' in initialization
     constexpr auto V = S<N>{};

When V's parameter does not depend on T, the program compiles as expected. This
affects GCC 5.x and 6.x in C++14 mode.