一段泛型约束的代码如下： ```cpp template requires std::integral void f(T&& t){ std::cout

@"WLGBC"#p8286 看标准是直接套用了[is_integral](https://zh.cppreference.com/w/cpp/types/is_integral)

C++ integral约束与&&参数的关系

WLGBC

一段泛型约束的代码如下：

template<typename T>
    requires std::integral<T>
void f(T&& t){
    std::cout << t << '\n';
}

int main() {
    int t = 10;
    f(10);              // OK
    f(std::move(t));    // OK
    f(t);               // Error
}

在不加requires std::integral<T>约束之前，很明显的是T&&构成万能引用，目的是想让左值和右值都能使用这个函数

但在加上整型约束后，万能引用似乎退变为右值引用了，仅右值可以调用

虽然说整型拷贝的代价不大，但我就是想要使用万能引用的话该怎么写？

以及此处为什么不是万能引用？

WLGBC

WLGBC 好吧，我突然想明白了，自问自答一波

template<typename T>
    requires std::integral<std::remove_reference_t<T>>
void f(T&& t)
{
    std::cout << std::boolalpha << std::is_same<T&&, int&&>() << '\n';
}

int main() {
    int t = 10;
    f(10);              // OK
    f(std::move(t));    // OK
    f(t);               // Error
}

只要让T套上std::remove_reference_t<>就好了，去除引用就能判断了，万能引用也没有失效，只是int&不符合泛型约束

题外话：

不得不说Clang的提示是最人性化的，MSVC提示过于简略，GCC提示略显复杂

MSVC

1>C:\dev\CppProject1\CppProject1\源.cpp(22,8): error C2672: “f”: 未找到匹配的重载函数
1>C:\dev\CppProject1\CppProject1\源.cpp(13,6): message : 可能是“void f(T &&)”
1>C:\dev\CppProject1\CppProject1\源.cpp(22,5): message : “f”: 未满足关联约束
1>C:\dev\CppProject1\CppProject1\源.cpp(13): message : 参见“f”的声明

GCC(MinGW)

1.cpp: In function 'int main()':
1.cpp:14:10: error: no matching function for call to 'f(int&)'
   14 |         f(t);               // Error
      |         ~^~~
1.cpp:6:6: note: candidate: 'template<class T>  requires  integral<T> void f(T&&)'
    6 | void f(T&& t){
      |      ^
1.cpp:6:6: note:   template argument deduction/substitution failed:
1.cpp:6:6: note: constraints not satisfied
In file included from 1.cpp:1:
c:\users\hex57\scoop\apps\mingw-winlibs\current\include\c++\12.2.0\concepts: In substitution of 'template<class T>  requires  integral<T> void f(T&&) [with T = int&]':
1.cpp:14:3:   required from here
c:\users\hex57\scoop\apps\mingw-winlibs\current\include\c++\12.2.0\concepts:100:13:   required for the satisfaction of
integral<T>' [with T = int&]
c:\users\hex57\scoop\apps\mingw-winlibs\current\include\c++\12.2.0\concepts:100:24: note: the expression 'is_integral_v<_Tp> [with _Tp = int&]' evaluated to 'false'
  100 |     concept integral = is_integral_v<_Tp>;
      |                        ^~~~~~~~~~~~~~~~~~

Clang(MSVC)

1.cpp:14:2: error: no matching function for call to 'f'
        f(t);               // Error
        ^
1.cpp:6:6: note: candidate template ignored: constraints not satisfied [with T = int &]
void f(T&& t){
     ^
1.cpp:5:14: note: because 'int &' does not satisfy 'integral'
    requires std::integral<T>
             ^
C:\Program Files\Microsoft Visual Studio\2022\Preview\VC\Tools\MSVC\14.34.31932\include\concepts:81:20: note: because
      'is_integral_v<int &>' evaluated to false
concept integral = is_integral_v<_Ty>;
                   ^
1 error generated.

c0nstexpr

WLGBC 看标准是直接套用了is_integral