[Bug c++/101904] Wrong result of decltype during instantiation of std::result_of

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101904

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Jonathan Wakely from comment #3)
> I think Clang doesn't even bother to instantiate the return type for the
> non-const overload, because it wouldn't be viable. I'm not sure if that's a
> conforming implementation or not.

[temp.inst] p9:

"If the function selected by overload resolution (12.2) can be determined
without instantiating a class template definition, it is unspecified whether
that instantiation actually takes place."

So the compiler is permitted to skip the result_of instantiation in the return
type if it knows that overload won't be selected anyway. But it's not required
to skip it, so your code is still wrong.