Why the template parameter should be used here for function call of decltype when define a template function with a trailing return type?

when using a trailing return type in C++ template function, the function call in decltype with template parameter as follows will compile successfully.

// case 1: no error
template <typename T>
struct basic_type { using type = T; };

template<typename T>
auto getBox1() -> decltype((getBox(basic_type<T>{})))
{
    return getBox(basic_type<T>{});
}

if the parameter of function getBox is void, a gramma error would assert immediately.

// case 2: error
template <typename T>
struct basic_type { using type = T; };

template<typename T>
auto getBox1() -> decltype((getBox())) // ERROR TIPS, "identifier "getBox" is undefined"
{
    return getBox();
}

since getBox function is not declared or defined either case, why case 1 is OK but case 2 would give an error?

Any explanation would be great appreciated.

googled this problem, but the seach engine could even not parse the content.