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?
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.
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.
