Inconsistent results of is_assignable<> [duplicate]

Answer 1

is_assignable::value is defined as true when declval() = declval() is well-formed and declval is defined as a function returning add_rvalue_reference::type.

We must remember that an assignment is only valid for a modifiable lvalue as the left operand. Also remember the rules of reference collapsing (especially the last two):

T&  &  -> T&
T&& &  -> T&
T&  && -> T&
T&& && -> T&&

So each case:

1. is_assignable and is_assignable

   Can we assign the result of a function returning an rvalue reference (an xvalue) to the result of another function returning an rvalue reference (another xvalue). No we can't. This should be false.

2. std::is_assignable and std::is_assignable

   Can we assign the result of a function returning an rvalue reference (an xvalue) to the result of a function returning an lvalue reference (an lvalue). We sure can. This should be true.

3. std::is_assignable and std::is_assignable

   Can we assign the result of a function returning an lvalue reference (an lvalue) to the result of a function returning an rvalue reference (an xvalue). No we can't. This should be false.

So I say GCC is right here.