[Bug c++/66601] New: RFE: improve diagnostics for failure to deduce template parameter pack that is not in the last position in the parameter list

[avi@cloudius-systems.com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66601

            Bug ID: 66601
           Summary: RFE: improve diagnostics for failure to deduce
                    template parameter pack that is not in the last
                    position in the parameter list
           Product: gcc
           Version: 5.1.1
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: avi@cloudius-systems.com
  Target Milestone: ---

Given the source

 template <typename... X, typename Y>
 void f(X... x, Y y) {
 }

 int main(int ac, char** av) {
   f(1, 2, 3, 4);
   return 0;
 }

gcc (correctly) fails to deduce X as int, int, int.

However, the diagnostic is not very helpful:

./variadic-function-nonlast.cc: In function ‘int main(int, char**)’:
./variadic-function-nonlast.cc:9:15: error: no matching function for call to
‘f(int, int, int, int)’
   f(1, 2, 3, 4);
               ^
./variadic-function-nonlast.cc:4:6: note: candidate: template<class ... X,
class Y> void f(X ..., Y)
 void f(X... x, Y y) {
      ^
./variadic-function-nonlast.cc:4:6: note:   template argument
deduction/substitution failed:
./variadic-function-nonlast.cc:9:15: note:   candidate expects 1 argument, 4
provided
   f(1, 2, 3, 4);


The user cannot understand from this why gcc is refusing to to deduce X, or
that explicitly specifying it (f<int, int, int>(1, 2, 3, 4)) would work.

If gcc would add an additional note:

./variadic-function-nonlast.cc:9:15: note:   parameter pack 'X' in non-last
position not specified; deduced as empty parameter pack 

then the user would understand exactly what happened here, and how to rectify
it.

The error message from clang 3.5 are not particularly helpful.