On Wed, May 21, 2025 at 12:54 AM Thiago Macieira wrote:
>
> decltype(std::declval<T (*)()>()())


That's clever. But every modern C++ compiler fails to compile the
following source file:

    #include <string> // string
    #include <utility> // declval

    template<typename T>
    decltype(auto) consteval Func(void) noexcept
    {
        return std::declval<T(*)(void)>()();
    }

    int main(void)
    {
        typedef decltype(Func<std::string>()) MyType;
    }

The problematic line is the return statement from 'Func', as the
compiler thinks that the result of 'declval' is being used in an
evaluated context.

The solution, I think, is to apply an attribute to 'Func' as follows:

    template<typename T>
    [[unevalutated]] decltype(auto) consteval Func(void) noexcept
    {
        return std::declval<T(*)(void)>()();
    }

This attribute will have two effects:
    (1) It will disallow the compiler to call this function in an
evaluated context
    (2) It will allow unrestricted use of "declval" inside the body of
the function