Template function type deduction, and return type

Question

Why a is true, and b is false? Or in other words why T in foo1 is int const but return type of foo2 is just int?

template<typename T>
constexpr bool foo1(T &) {
    return std::is_const<T>::value;
}

template<typename T>
T foo2(T &);

int main() {
    int const x = 0;
    constexpr bool a = foo1(x);
    constexpr bool b = std::is_const<decltype(foo2(x))>::value;
}

Answer 1

const-qualifiers are ignored if function returned type is non-class, non-array type. If you use some class instead of plain int it will produce 1 1:

  struct Bar{};

  int main()
  {
     Bar const x{};
     constexpr bool a = foo1(x);
     constexpr bool b = std::is_const<decltype(foo2(x))>::value;
  }

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