[Bug c++/71843] New: [concepts] Diagnostics issued for constraint satisfaction failure fail to elucidate unsatisfied constraints

[tom at honermann dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71843

            Bug ID: 71843
           Summary: [concepts] Diagnostics issued for constraint
                    satisfaction failure fail to elucidate unsatisfied
                    constraints
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: tom at honermann dot net
                CC: andrew.n.sutton at gmail dot com, asutton at gcc dot gnu.org
  Target Milestone: ---

The diagnostics emitted when overload resolution fails due to constraint
satisfaction failure do not elucidate the constraints that were not satisfied. 
For non-trivial concepts, it can be difficult and tedious for a user to
determine exactly why a given type did not satisfy a set of constraints.

For example, using gcc 6.1.0:

$ cat t.cpp
template<typename T>
concept bool C = requires(T t) { t.i; };
template<C T>
auto f(T t) {
    return t.i;
}
struct S {
};
auto x = f(S{});

$ g++ --version
g++ (GCC) 6.1.0
...

$ g++ -c -std=c++1z -fconcepts t.cpp
t.cpp:9:15: error: cannot call function ‘auto f(T) [with T = S]’
 auto x = f(S{});
               ^
t.cpp:4:6: note:   constraints not satisfied
 auto f(T t) {
      ^
t.cpp:4:6: note:   concept ‘C<S>’ was not satisfied

The error message clearly indicates that S does not satisfy the constraints
associated with concept C, but does not explain that the reason constraint
satisfaction failed is because the expression t.i is ill-formed for an object t
of type S. 

When designing types intended to model a particular concept, it is often useful
to statically assert that the type models the intended concept.  Here too the
error messages are less helpful then desired:

$ cat t2.cpp
template<typename T>
concept bool C = requires(T t) { t.i; };
struct S {
};
static_assert(C<S>);

$ g++ -c -std=c++1z -fconcepts t2.cpp
t2.cpp:5:1: error: static assertion failed
 static_assert(C<S>);
 ^~~~~~~~~~~~~