Following up on Nawaz's link, the remaining questions are answered by N3690 §7.1.6.4/11:
If the type of an entity with an undeduced placeholder type is needed to determine the type of an expression, the program is ill-formed.
This means that even if SFINAE worked with return type deduction, it couldn't be used to query one function declaration from another. The signature is essentially invalid until the return
statement is processed, which occurs at the closing brace of the class {}
definition, and after the definitions of preceding members have been processed.
In a sense, all member decltype(auto)
functions are incomplete with respect to preceding functions in the same class:
struct s {
void f() { a(); } // error: use of ‘auto s::a()’ before deduction of ‘auto’
auto a() { return 3; }
};
This is GCC's complaint; it goes away if the member declarations are reversed. This is because the function definitions are processed in order of declaration, when the }
from the class definition is reached. If the statement a();
is processed before the return 3;
, then the program is ill-formed.