[basic.types#general-example-3]

extern int arr[];               // the type of arr is incomplete
typedef int UNKA[];             // UNKA is an incomplete type
UNKA* arrp;                     // arrp is a pointer to an incomplete type
int  arr[10];                   // now the type of arr is complete
void bar() {
  arrp = &arr;                  // error: different types
}

The above example is a simplified example of [basic.types#general-example-3], the comment says it's an error. However, it's valid even though their types are different. A qualification conversion can be used to convert the type of &arr to the cv-unqualified type of arrp, which obeys [expr.ass#3]

If the right operand is an expression, it is implicitly converted to the cv-unqualified type of the left operand.

The type of &arr is pointer to array of 10 int whilst the type of arrp is pointer to array of unknown bound of int, as per [conv.qual#3], the former type can be converted to the later type by a qualification conversion. Hence, the comment should be "ok". GCC is in compliance with these rules.