Why do Clang and MSVC not like a member typedef declaration with a redundant set of parentheses?

Question

Consider

using foo = int;

struct A {
    typedef A (foo)();
};

GCC and ICC accept the snippet, while Clang and MSVC reject it. Clang's error message is

<source>:4:15: error: function cannot return function type 'void ()'
    typedef A (foo)();
              ^
<source>:4:13: error: typedef name must be an identifier
    typedef A (foo)();
            ^
2 errors generated.

And MSVC says

<source>(4,15): error C2091: function returns function
    typedef A (foo)();
              ^

(live demo)

Why do Clang and MSVC produce this error? Which compilers are correct?

(I'm specifically looking for quotation from the standard or any defect report.)

Answer 1

Both Clang and MSVC are ignoring the typedef specifier and reading the declaration as that of a constructor (that is, A is the constructor name) accepting parameter types (foo) (that is, (int)) and "returning" a function type signified by the trailing parentheses ().

Yes, constructors don't have return types; but if they did have return types they would have return type A, so the additional () at the end makes these compilers think that you now have a constructor with return type the function type A().

This is supported by noting that the following "similar" declarations have similar error messages:

A (foo)();
typedef ~A(foo)();

Also, by adding static we can get an illuminating error message from MSVC:

A static (int)();
error C2574: '(__cdecl *A::A(int))(void)': cannot be declared static

For workarounds: under Clang (but not MSVC) you can move the typedef specifier to the right, or use an elaborated type specifier:

A typedef (foo)();
typedef struct A (foo)();

Under all compilers you can remove or add parentheses:

typedef A foo();
typedef A ((foo))();

And you can always update to a type alias:

using foo = A();