Function default argument value depending on argument name in C++

Question

If one defines a new variable in C++, then the name of the variable can be used in the initialization expression, for example:

int x = sizeof(x);

And what about default value of a function argument? Is it allowed there to reference the argument by its name? For example:

void f(int y = sizeof(y)) {}

This function is accepted in Clang, but rejected in GCC with the error:

'y' was not declared in this scope

Demo: https://gcc.godbolt.org/z/YsvYnhjTb

Which compiler is right here?

Answer 1

According to the C++17 standard (11.3.6 Default arguments)

9 A default argument is evaluated each time the function is called with no argument for the corresponding parameter. A parameter shall not appear as a potentially-evaluated expression in a default argument. Parameters of a function declared before a default argument are in scope and can hide namespace and class member name

It provides the following example:

int h(int a, int b = sizeof(a)); // OK, unevaluated operand

So, this function declaration

void f(int y = sizeof(y)) {}

is correct because, in this expression sizeof(y), y is not an evaluated operand, based on C++17 8.3.3 Sizeof:

1 The sizeof operator yields the number of bytes in the object representation of its operand. The operand is either an expression, which is an unevaluated operand (Clause 8), or a parenthesized type-id.

and C++17 6.3.2 Point of declaration:

1 The point of declaration for a name is immediately after its complete declarator (Clause 11) and before its initializer (if any), except as noted below.

Answer 2

The code does not appear ill-formed, so Clang is alright.

[basic.scope.pdecl]

1 The point of declaration for a name is immediately after its complete declarator ([dcl.decl]) and before its initializer (if any), except as noted below.

This is the notorious passage that is under discussion. I bring it here just to mention that "except as noted below" doesn't include any mention of default arguments. So y is declared right before = sizeof(y).

The other relevant paragraph is

[dcl.fct.default]

9 A default argument is evaluated each time the function is called with no argument for the corresponding parameter. A parameter shall not appear as a potentially-evaluated expression in a default argument. Parameters of a function declared before a default argument are in scope and can hide namespace and class member names.

sizeof(y) is not potentially evaluated, so this is also fine.

Seeing as the first paragraph makes y available as a name, and it's used in a way that is not illegal, must be some quirk of GCC that rejects the code.

Though personally, I don't see it as a great loss. This is not the most practical bit of code.