新简化！typename 在 C++20 不再必要

If X::Y — where T is a template parameter — is to denote a type, it must be preceded by the keyword typename; otherwise, it is assumed to denote a name producing an expression. There are currently two notable exceptions to this rule: base-specifiers and mem-initializer-ids. For example:

template<class T>
struct D: T::B { // No typename required here.
};

Clearly, no typename is needed for this base-specifier because nothing but a type is possible in that context. 

1#include <iostream>
2
3struct S {
4    using X = int;
5    using R = char;
6};
7
8
9// 类模板
10template<class T,
11         auto F = typename T::X{}>    // 需要typename
12struct Foo {       
13    using X2 = T::X;                  // 不需要typename
14    typedef T::X X3;                  // 不需要typename                 
15    T::X val;                         // 不需要typename
16
17    T::R f(T::X x) {                  // 不需要typename
18        typename T::X i;              // 需要typename
19        return static_cast<T::R>(x);  // 不需要typename
20    }                                 
21
22    auto g() -> T::R {                // 不需要typename
23    }                                 
24
25    template<class U = T::X>          // 不需要typename
26    void k(U);                        
27};                                    
28
29
30// 函数模板                           
31template<class T>                     
32T::R                                  // 不需要typename
33f(typename T::X x)                    // 需要typename
34{                                     
35    using X2 = T::X;                  // 不需要typename
36    typedef typename T::X X3;         // 需要typename
37    typename T::X val;                // 需要typename
38
39    typename T::R f();                // 需要typename
40    auto g() -> T::R;                 // 不需要typename
41    void k(typename T::X);            // 需要typename
42
43    auto lam = [](T::X) {};           // 不需要typename
44
45    return static_cast<T::R>(x);      // 不需要typename
46}
47
48int main()
49{
50    Foo<S> foo;
51    f<S>(65);
52
53    return 0;
54}