Can dangling pointer be equal to valid pointer during constant evaluation in C++?

During constant expression evaluation in C++17, shall the compiler consider any pointer addressing a valid object unequal to any pointer addressing an object after the end of its lifetime?

For example:

constexpr auto f(int * x = nullptr) {
    int c = 0;
    auto p = &c;
    if ( x == p )
        throw "unexpected";
    return p;
};

int main() {
    static_assert( f( f() ) );
}

Here the inner call of function f() returns a dangling pointer, which is passed to f again. I believed that the condition x == p must be false since x is a dangling pointer and p is a valid pointer, and it is indeed so in Clang. But in GCC the condition is satisfied and the constant evaluation fails due to throw. Demo: https://gcc.godbolt.org/z/ehcMro17q

Is it an undefined or implementation-defined behavior, or one of the compilers is wrong?