DerivedFrom should be publicly and unambiguously

[ericniebler]

Proposed Resolution

Change [concepts.lib.corelang.derived] as follows:

 template <class T, class U>
 concept bool DerivedFrom() {
-  return see below ;
+  return is_base_of<U, T>::value &&
+    is_convertible<remove_cv_t<T>*, remove_cv_t<U>*>::value; // see below
 }

-1 DerivedFrom<T, U>() is satisfied if and only if is_base_of<U, T>::value is
-  true.
+1 There need not be a subsumption relationship between DerivedFrom<T, U>() and either
+  is_base_of<U, T>::value or is_convertible<remove_cv_t<T>*, remove_cv_t<U>*>::value.
+
+2 [Note: DerivedFrom<T, U>() is satisfied if and only if T is publicly and unambiguously
+  derived from U, or T and U are the same class type ignoring cv qualifiers.-end note]

[ericniebler]

@CaseyCarter Comments?

[CaseyCarter]

"if and only if is_base_of<U, T>::value && is_convertible<T*, U*>::value is true" is shorter/simpler.

[ericniebler]

Resolution edited to hide the subsumption relationship with the traits (#321).

[timsong-cpp]

is_base_of<U, T> ignores cv-qualification, but is_convertible<T*, U*> doesn't. Should DerivedFrom care about cv-qualifications on T and U?

[CaseyCarter]

Should DerivedFrom care about cv-qualifications on T and U?

It definitely should not; PR corrected to use is_convertible<remove_cv_t<T>*, remove_cv_t<U>*>::value.