Sizeof applied to parameter of incomplete array type

Question

For this code:

int f(int x[])
{
    return sizeof x;
}

GCC produces:

warning: 'sizeof' on array function parameter 'x' will return size of 'int *'

Clang produces:

warning: sizeof on array function parameter will return size of 'int *' instead of 'int[]'

Question: if x has incomplete type (since the size is not present), then is it expected to have:

error: invalid application of 'sizeof' to incomplete type

Extra: does it mean that return size of 'int *' is a GCC/Clang extension?

Answer 1

An array as an argument to a function is actually a pointer. From section 6.7.6.3p7 of the C standard regarding Function Declarators:

A declaration of a parameter as ‘‘array of type’’ shall be adjusted to ‘‘qualified pointer to type’’, where the type qualifiers (if any) are those specified within the [ and ] of the array type derivation. If the keyword static also appears within the [ and ] of the array type derivation, then for each call to the function, the value of the corresponding actual argument shall provide access to the first element of an array with at least as many elements as specified by the size expression.

This means that this:

int f(int x[])
{
    return sizeof x;
}

Is exactly the same as this:

int f(int *x)
{
    return sizeof x;
}

So x does have complete type and there is no error. Both gcc and clang are being helpful in generating a warning as the result might not be what one expects.