Is GCC or Clang correct about the name of a function parameter being in scope in its own default argument?

I think clang is correct. From basic.scope.pdecl:

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

int x = 12;{ int x = x; }

Here the second x is initialized with its own (indeterminate) value. — end example ]

Also, from dcl.fct.default:

Default arguments are evaluated each time the function is called. The order of evaluation of function arguments is unspecified. Consequently, parameters of a function shall not be used in a default argument, even if they are not evaluated. Parameters of a function declared before a default argument are in scope and can hide namespace and class member names