https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64232
--- Comment #2 from Michael Mehlich <mmehlich at semanticdesigns dot com> --- My expectation was that the compiler declares the copy assignment operator of Y as deleted as its base class X is not assignable; std::is_assignable would then just have to look at the copy constructor for Y and notice that it is deleted, which would obviously be in the immediate context. But arguably, 12.8 paragraph 34 item 4 only looks at the copy assignment operator for X, not whether X can actually be copy-assigned, which - as the example above shows - may require that the copy constructor for X is not deleted. By the way, std::is_nothrow_assign has the same problem (i.e. declaring X(const X&) as throwing, and operator=(X x) as not throwing results in gcc claiming that the copy assignment operator for Y is not throwing).
