This is covered in a note to 6.5.1 Primary expressions:
2 - An identifier is a primary expression, provided it has been declared as designating an object (in which case it is an lvalue) or a function (in which case it is a function designator). 79)
79) Thus, an undeclared identifier is a violation of the syntax.
A conforming implementation is required by 5.1.1.3 Diagnostics to produce a diagnostic message in response to the syntax violation of a function call expression involving an undeclared identifier as the expression denoting the called function. It is of course free to proceed to compile the program as if the identifier had been declared in the implicit-int
C89 style.
The paragraph 6.5.2.2p5 must be read with reference to the constraint 6.5.2.2p1:
1 - The expression that denotes the called function shall have type pointer to function returning
void
or returning an object type other than an array type.
So, if the "expression that denotes the called function" does not have type "pointer to function returning an object type", it must ipso facto (by the constraint 6.5.2.2p1) have type "pointer to function returning void
", and it is this case which the "Otherwise" in 6.5.2.2p5 covers. That is, with my insertion in [square brackets]:
5 - If the expression that denotes the called function has type pointer to function returning an object type, the function call expression has the same type as that object type, and has the value determined as specified in 6.8.6.4. Otherwise, [i.e. if the expression that denotes the called function has type pointer to function returning
void
,] the function call has typevoid
.
This is a case of special language being required for void
as opposed to object types; is not a licence for the called function expression to be or contain an undeclared identifier.