Dependent lookup in decltype on return type fails to compile

I've been playing with trailing return types that resolve to the same type, with a different expression, but resolves to something legal. This works:

template <typename>
struct Cls {
  static std::size_t f();
};

template <typename T>
auto Cls<T>::f() -> decltype(std::size_t{}) { return 0; }

But if I change the definition to something that should be equivalent, it fails

template <typename T>
auto Cls<T>::f() -> decltype(sizeof(T)) { return 0; }

clang's error (gcc is similar):

error: return type of out-of-line definition of 'Cls::f' differs from that in the declaration
auto Cls<T>::f() -> decltype(sizeof(T)) { return 0; }
             ^
note: previous declaration is here
  static std::size_t f();
         ~~~~~~~~~~~ ^

If I replace sizeof(T) with sizeof(int) it compiles successfully. Why is it illegal with a dependent type only?