用概念检查属性的类型

Question

我想检查 struct/class 的属性是否符合我对概念的需求，但编译器抱怨。

示例：

struct N
{
    char value;
    auto Get() { return value; }
};

struct M
{
    int value;
    auto Get() { return value; }
};

void func3( auto n )
    requires requires
{
    //{ n.Get() } -> std::same_as<int>;
    { n.value } -> std::same_as<int>;
}
{
    std::cout << __PRETTY_FUNCTION__ << std::endl;
}


void func3( auto n )
    requires requires 
{
    //{ n.Get() } -> std::same_as<char>;
    { n.value } -> std::same_as<char>;
}
{
    std::cout << __PRETTY_FUNCTION__ << std::endl;
}

int main()
{
    M m;
    N n;

    func3( n );
    func3( m );
}

使用 ( gcc 10.1.1 )

产生了更长的一堆消息

main.cpp: In function 'int main()':
main.cpp:202:18: error: no matching function for call to 'func3(N&)'
  202 |         func3( n );
      |                  ^
main.cpp:154:10: note: candidate: 'void func3(auto:15) requires requires{{func3::n.value} -> decltype(auto) [requires std::same_as<<placeholder>, int>];} [with auto:15 = N]'
  154 |     void func3( auto n )
      |          ^~~~~
main.cpp:154:10: note: constraints not satisfied
main.cpp: In instantiation of 'void func3(auto:15) requires requires{{func3::n.value} -> decltype(auto) [requires std::same_as<<placeholder>, int>];} [with auto:15 = N]':
main.cpp:202:18:   required from here
main.cpp:154:10:   required by the constraints of 'template<class auto:15> void func3(auto:15) requires requires{{func3::n.value} -> decltype(auto) [requires std::same_as<<placeholder>, int>];}'
main.cpp:155:18:   in requirements  [with auto:15 = N]
main.cpp:158:13: note: 'n.value' does not satisfy return-type-requirement
  158 |         { n.value } -> std::same_as<int>;
      |           ~~^~~~~
cc1plus: note: set '-fconcepts-diagnostics-depth=' to at least 2 for more detail
main.cpp:165:10: note: candidate: 'void func3(auto:16) requires requires{{func3::n.value} -> decltype(auto) [requires std::same_as<<placeholder>, char>];} [with auto:16 = N]'
  165 |     void func3( auto n ) 
      |          ^~~~~
main.cpp:165:10: note: constraints not satisfied
main.cpp: In instantiation of 'void func3(auto:16) requires requires{{func3::n.value} -> decltype(auto) [requires std::same_as<<placeholder>, char>];} [with auto:16 = N]':
main.cpp:202:18:   required from here
main.cpp:165:10:   required by the constraints of 'template<class auto:16> void func3(auto:16) requires requires{{func3::n.value} -> decltype(auto) [requires std::same_as<<placeholder>, char>];}'
main.cpp:166:18:   in requirements  [with auto:16 = N]
main.cpp:169:13: note: 'n.value' does not satisfy return-type-requirement
  169 |         { n.value } -> std::same_as<char>;
      |           ~~^~~~~
main.cpp:203:18: error: no matching function for call to 'func3(M&)'
  203 |         func3( m );
      |                  ^   
main.cpp:154:10: note: candidate: 'void func3(auto:15) requires requires{{func3::n.value} -> decltype(auto) [requires std::same_as<<placeholder>, int>];} [with auto:15 = M]'
  154 |     void func3( auto n ) 
      |          ^~~~~
main.cpp:154:10: note: constraints not satisfied
main.cpp: In instantiation of 'void func3(auto:15) requires requires{{func3::n.value} -> decltype(auto) [requires std::same_as<<placeholder>, int>];} [with auto:15 = M]':
main.cpp:203:18:   required from here

检查 Get() 函数的 return 类型的版本按预期工作。这里有什么问题？

见 compiler explorer

clang：按预期工作
gcc 10.1.1 失败并显示错误消息
gcc 主干：ICE！瑞银：-

更新（11 月 21 日 12 日）

gcc 主干（版本 12.x.x。）有效

似乎有人修复了这个错误： bug report

Answer 1

GCC 实际上是正确的（当拒绝代码时，不要发疯）。引用标准

[expr.prim.req.compound]/1.3

If the return-type-requirement is present, then:

Substitution of template arguments (if any) into the return-type-requirement is performed.

The immediately-declared constraint ([temp.param]) of the type-constraint for decltype((E)) shall be satisfied.

E是我们的表达式，即n.value.

现在，decltype(n.value) 是 char 或 int，那是因为 decltype has a special rule for class member access and id expressions。但是 decltype((n.value)) 是 char& 或 int&。值类别在处理一般表达式（如带括号的class成员访问）时编码为decltype类型。

当我们修改您的示例时，您的示例在 GCC 中有效

void func3( auto n )
    requires requires
{
    //{ n.Get() } -> std::same_as<int>;
    { n.value } -> std::same_as<int&>;
}
{
    std::cout << __PRETTY_FUNCTION__ << std::endl;
}


void func3( auto n )
    requires requires 
{
    //{ n.Get() } -> std::same_as<char>;
    { n.value } -> std::same_as<char&>;
}
{
    std::cout << __PRETTY_FUNCTION__ << std::endl;
}

Answer 2

根据[expr.prim.req.compound]/1.3

If the return-type-requirement is present, then:
Substitution of template arguments (if any) into the return-type-requirement is performed.
The immediately-declared constraint ([temp.param]) of the type-constraint for decltype((E)) shall be satisfied. [Example: Given concepts C and D,
requires {
  { E1 } -> C;
  { E2 } -> D<A1, ⋯, An>;
};
is equivalent to
requires {
  E1; requires C<decltype((E1))>;
  E2; requires D<decltype((E2)), A1, ⋯, An>;
};
(including in the case where n is zero). — end example ]

并且，从 [dcl.type.decltype]/1, particularly [dcl.type.decltype]/1.5 适用

For an expression E, the type denoted by decltype(E) is defined as follows:

[...]

(1.5) otherwise, if E is an lvalue, decltype(E) is T&, where T is the type of E;

因此，由于 value 是 { n.value } -> std::same_as<int>; 表达式中的 lvalue，return-type-requirement -> std::same_as<...>; for the { n.value } expression need to be matched for return type int&:

#include <iostream>
#include <type_traits>

template <typename T> struct Foo {
  T value;
  auto getValue() { return value; }
};

using FooInt = Foo<int>;
using FooChar = Foo<char>;

void abbreviatedFunctionTemplate(auto n) requires requires {
  { n.value } -> std::same_as<int&>; }
{ std::cout << "int overload\n" << std::endl; }

void abbreviatedFunctionTemplate(auto n) requires requires {
  { n.getValue() } ->std::same_as<char>; }
{ std::cout << "char overload\n" << std::endl; }

int main() {
  abbreviatedFunctionTemplate(FooInt{});  // int overload
  abbreviatedFunctionTemplate(FooChar{}); // char overload
}

用概念检查属性的类型

checking type of attribute with concepts

c++

c++-concepts

c++20