[llvm-bugs] [Bug 26297] New: Clang incorrectly rejects pointers-to-incomplete-classes as covariant return types when dropping cv-qualifiers

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=26297

            Bug ID: 26297
           Summary: Clang incorrectly rejects
                    pointers-to-incomplete-classes as covariant return
                    types when dropping cv-qualifiers
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: normal
          Priority: P
         Component: -New Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: sfinae at hotmail.com
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

Created attachment 15709
  --> https://llvm.org/bugs/attachment.cgi?id=15709&action=edit
Minimal self-contained repro

struct Incomplete;

struct Base {
    virtual const Incomplete * meow() = 0;
};

struct Derived : Base {
    virtual Incomplete * meow() override { return nullptr; }
};

int main() { }

/*** N4567 10.3 [class.virtual]/8:
"If the class type in the covariant return type of D::f differs from that of
B::f,
the class type in the return type of D::f shall be complete at the point of
declaration of D::f
or shall be the class type D." ***/

Accepted by GCC 5.2.0, VS 2015, and EDG 4.10.1.

Rejected by Clang 3.7.0 (and trunk on http://melpon.org/wandbox/ ) with:

prog.cc:8:26: error: return type of virtual function 'meow' is not covariant
with the return type of the function it overrides ('Incomplete' is incomplete)
    virtual Incomplete * meow() override { return nullptr; }
                         ^
prog.cc:1:8: note: forward declaration of 'Incomplete'
struct Incomplete;
       ^

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