[Bug c++/48934] New: no rejection reason given for SFINAE

["redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48934

           Summary: no rejection reason given for SFINAE
           Product: gcc
           Version: 4.6.0
            Status: UNCONFIRMED
          Keywords: diagnostic
          Severity: enhancement
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: redi@gcc.gnu.org
                CC: froydnj@gcc.gnu.org


int foo(int);

template<typename T>
  struct sfinae
  { };

template<>
  struct sfinae<float>
  {
      typedef double type;
  };

template<typename T>
  typename sfinae<T>::type
  foo(T t)
  { return t; }

struct Bar { };

Bar b = foo( Bar() );


The error prints the two candidate functions and reason they weren't viable (I
love this feature, thanks Nathan!)

reason.cc:20:20: error: no matching function for call to 'foo(Bar)'
reason.cc:20:20: note: candidates are:
reason.cc:1:5: note: int foo(int)
reason.cc:1:5: note:   no known conversion for argument 1 from 'Bar' to 'int'
reason.cc:15:10: note: template<class T> typename sfinae::type foo(T)

But no reason is given for the template.

I think the reason Clang++ gives is pretty good:
note: candidate template ignored: substitution failure [with T = Bar]

The key points to include in the reason are the template arguments and that
substitution failed (more formally, type deduction failed because substitution
resulted in an invalid type)

I've only checked this with 4.6 so apologies if it's already been improved on
trunk.