checking type of attribute with concepts

I want to check that an attribute of a struct/class fits to my needs with concepts, but compiler complains about.

Example:

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 );
}

Results in a bit longer bunch of messages with ( gcc 10.1.1 )

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

The version with checking the return type of the Get() function works as expected. What is wrong here?

See on compiler explorer

clang: works as expected
gcc 10.1.1 fails with error message
gcc trunk: ICE! ubs :-

Update ( 12. Nov. 21 )

gcc trunk ( version 12.x.x.) works

It seems someone has fixed the bug: bug report

Solution

GCC is actually right (when rejecting the code, not going bonkers). To quote the standard

[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 is our expression, namely n.value.

Now, decltype(n.value) is char or int, that's because decltype has a special rule for class member access and id expressions. But decltype((n.value)) is char& or int&. The value category is encoded in the type of decltype when dealing with a general expression (such as a parenthesized class member access).

Your example works in GCC when we amend it

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;
}