[expr.prim.this]/2 alludes to the grammar production `declarator` without necessity

[jabelloc]

[expr.prim.this]/2:

If a declaration declares a member function or member function template of a class X, the expression this is a prvalue of type “pointer to cv-qualifier-seq X” between the optional cv-qualifier-seq and the end of the function-definition, member-declarator, or declarator. . . .

A member-declarator is already defined in terms of a declarator. Therefore, the declarator can be eliminated above without loss of generality.

[cpplearner]

In template auto A::f(int t) -> decltype(t + g());, A::f(int t) -> decltype(t + g()) is a declarator but not a member-declarator.

[jensmaurer]

Right you are.

[cpplearner]

(But declarator should probably be changed to init-declarator, to make this valid in a trailing requires-clause.)