[Bug c++/71007] New: Divergence between treatment of char[0] between OR (=> SFINAE failure) and diagnostic printing (no failure)

[schaub.johannes at googlemail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71007

            Bug ID: 71007
           Summary: Divergence between treatment of char[0] between OR (=>
                    SFINAE failure) and diagnostic printing (no failure)
           Product: gcc
           Version: 6.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: schaub.johannes at googlemail dot com
  Target Milestone: ---

The following is supposed to tell the user that char[0] is an invalid type,
during diagnostic printing. But instead, it infinitely recurses up to SIZE_MAX
or something during printing the diagnostic message, eventually crashing

   #include <utility>
   #include <tuple>

   template<typename T, T ...I>
   auto ignore_n(std::integer_sequence<T, I...>) {
    return std::make_tuple((I, std::ignore)...);
   }

   template<typename... Ts>
   auto function(Ts... ts)
      -> decltype((std::tuple_cat(
           
ignore_n(std::make_index_sequence<sizeof(char[int(sizeof...(Ts))-1])-1>   ()),
            std::tuple<double, bool>()) = std::forward_as_tuple(ts...)),
void())
   {

   }

   int main() {
      function(2);
      function(1, 2, 3);
   }

Reduced test-case. Here it prints nothing when detailing on why "function" is
not callable:

   main.cpp: In function 'int main()':
   main.cpp:9:7: error: no matching function for call to 'f(char [1])'
       f(x);
       ^
   main.cpp:4:6: note: candidate: template<int N, class T> void f(char (&)[N])
    void f(char(&)[N])
      ^
   main.cpp:4:6: note:   template argument deduction/substitution failed: