Conceptualized `operator auto` in C++20

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Starting from C++20 we can precede auto keyword with the name of the concept to limit possible types. And in particular this combination is possible in class conversion operator auto, e.g.

template <typename T> concept x = true;

struct S
{
    operator x auto() { return 2; }
    operator auto() { return 1; }
};

int main() { return S{}.operator x auto(); }

But Clang is the only compiler that accepts the whole program, however main() returns 1 (and not 2 as I would expected), demo: https://gcc.godbolt.org/z/b16jYGa81

GCC accepts the struct definition, but refuses to compile S{}.operator x auto().

And MSVC refuses to accept even struct S with the error:

error C2535: 'S::operator auto(void)': member function already defined or declared

Just wonder, which of the compilers is right here (if any)?

auto c++-concepts c++20 c++
1 Answers

This conversion function:

operator auto() { return 1; }

Means exactly the same as this converison function:

operator int() { return 1; }

We're deducing the return type from 1, this isn't a function template.

This conversion function:

operator x auto() { return 2; }

Means roughly the same thing as:

operator int() { static_assert(x<int>); return 2; }

We're deducing the return type from 2 and ensuring that that type (int) satisfies a particular concept (x).

Putting both together, we have two functions (neither is a function template), both of which are named operator int(), and that's just not allowed. This should be ill-formed even at the point of declaration since the name operator int() is bound to two conflicting declarations.

Note that the second one is still named operator int(), not operator x auto().

user_20690640
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