Clang and GCC has an inconsistent interpretation to the case where the parameter name appears as an unevaluated operand

Question

#include <iostream>
void fun(int a = sizeof(a)){
    std::cout<< a<<std::endl;
}
int main(){
   fun();
}

Consider this case. Clang accepts it while GCC rejects it. According to [dcl.fct.default] p9

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.

[basic.def.odr] p2

An expression or conversion is potentially evaluated unless it is an unevaluated operand

sizeof is not a potentially-evaluated expression and a can be found at this point. I wonder which one is correct for this case?

Dup of [Using a parameter's name inside its own default value - is it legal?](https://stackoverflow.com/questions/40513156/using-a-parameters-name-inside-its-own-default-value-is-it-legal) and [Function default argument value depending on argument name in C++](https://stackoverflow.com/questions/69461415/function-default-argument-value-depending-on-argument-name-in-c) — Language Lawyer, Feb 10 '22 at 09:56

score 1 · Accepted Answer · answered Feb 10 '22 at 07:02

GCC is wrong. It assumes that a is not yet in scope in the default argument, although generally the point of declaration is immediately after the declarator before the initializer and none of the exceptions apply here. [basic.scope.pdecl]/1

An older bug report for this is here.

Note however, that the bug report has equivalent code to yours, but claims it should be ill-formed. That is because it was written before CWG 2082 which allowed the use of a parameter in an unevaluated context in the default argument.

Related question regarding this GCC bug here.

Clang and GCC has an inconsistent interpretation to the case where the parameter name appears as an unevaluated operand

1 Answers1