The Concepts TS (N4641) states the following in 7.1.6.4 [dcl.spec.auto] p4 (this section is now 10.1.7.4 in the WP):

"A constrained-type-specifier C1 within the declared return type of an abbreviated function template declaration does not designate a placeholder if its introduced constraint-expression (7.1.6.4.2) is determined to be equivalent, using the rules in 14.6.6.1 for comparing expressions, to the introduced constraint-expression for a constrained-type-specifier C2 in the parameter-declaration-clause of that function declaration. Instead, C1 is replaced by the template parameter invented for C2 (8.3.5)."

Gcc implements this behaviour (with an exception reported in bug 81139) for abbreviated functions that do not specify a trailing return type, but not for those that do:

$ cat t.cpp
template<typename> concept bool C = true;
template<C> class ct {};
ct<C> af1(C) { return ct<int>{}; }
auto af2(C) -> ct<C> { return ct<int>{}; }
auto v1 = af1('c'); // template instantiation fails.
auto v2 = af2('c'); // template instantiation succeeds.

$ g++ --version
g++ (GCC) 7.1.0
...

$ g++ -c -fconcepts t.cpp
t.cpp: In instantiation of ‘ct<C> af1(auto:1) [with auto:1 = char]’:
t.cpp:5:18:   required from here
t.cpp:3:31: error: could not convert ‘ct<int>{}’ from ‘ct<int>’ to ‘ct<char>’
 ct<C> af1(C) { return ct<int>{}; }
                               ^

The expected behaviour is that both template instantiations fail.