[Bug c++/81178] New: [concepts] poor (partial?) diagnostic for alias substitution failure in a concept body

[Casey at Carter dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81178

            Bug ID: 81178
           Summary: [concepts] poor (partial?) diagnostic for alias
                    substitution failure in a concept body
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: Casey at Carter dot net
  Target Milestone: ---

GCC 6.3/7.1/trunk as of 20170621 all diagnose this ill-formed program
(https://wandbox.org/permlink/uatmNSnlLbhvSvls):

template<class>
concept bool C1 = true;

template<class T>
using alias = typename T::type;

template<class T>
concept bool C2 = C1<alias<T>>;

void bar(C2) {}

int main() { bar(42); }

with:

prog.cc: In function 'int main()':
prog.cc:12:20: error: cannot call function 'void bar(auto:1) [with auto:1 =
int]'
 int main() { bar(42); }
                    ^
prog.cc:10:6: note:   constraints not satisfied
 void bar(C2) {}
      ^~~
prog.cc:10:6: note: in the expansion of concept 'C2<auto:1> template<class T>
concept const bool C2<T> [with T = int]'

(Yes, that is the entire diagnostic.) If we expand the definition of C2 fully
into bar (https://wandbox.org/permlink/GKW0vWSPIY5XrGRE):

template<class T>
  requires C1<alias<T>>
void bar(T) {}

the compiler gives a more complete diagnostic:

prog.cc: In function 'int main()':
prog.cc:14:20: error: cannot call function 'void bar(T) [with T = int]'
 int main() { bar(42); }
                    ^
prog.cc:12:6: note:   constraints not satisfied
 void bar(T) {}
      ^~~
prog.cc:12:6: note: invalid use of the concept 'C1<typename T::type>'
prog.cc:14:20: error: 'int' is not a class, struct, or union type
 int main() { bar(42); }
                    ^