The following code is accepted by GCC, Clang and MSVC. I thought it was ill-formed because the variable V0 is not usable in constant expressions. Is this just a bug in all of those compilers? Or is there an explanation in the C++ rules?

struct S { constexpr operator int() const { return 0; } };

void foo(S V0) { constexpr int V1 = V0; }

int main()
{
   S V0{};
   foo(V0);
}