mishandling of cv-qualification dropping in covariant return types

[zygoloid]

Bugzilla Link	17859
Version	trunk
OS	Linux
CC	@DougGregor

Extended Description

Clang accepts this ill-formed code:

typedef int X;
struct A { virtual const X *f(); };
struct B : A { virtual X *f(); };

Per 10.3/7, a covariant return type can only drop a cv-qualifier if it is a pointer or reference to class type.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: None (ec04fc15-fa35-46f2-80e1-5d271f2ef708)

| | | | --- | --- | | Bugzilla Link | [17859](https://llvm.org/bz17859) | | Version | trunk | | OS | Linux | | CC | @DougGregor |

Extended Description

Clang accepts this ill-formed code:

typedef int X;
struct A { virtual const X *f(); };
struct B : A { virtual X *f(); };

Per 10.3/7, a covariant return type can only drop a cv-qualifier if it is a pointer or reference to class type.

[ecnelises]

struct M {};
struct A { virtual const M *f(); };
struct B : A { virtual volatile M *f(); };

This should not be accepted but also acceptable to clang. GCC treats it as error.

As for motivating case:

struct A { virtual const int *f(); };
struct B : A { virtual int *f(); };

GCC emits warning instead of error, although it is ill-formed according to class.virtual:

The return type of an overriding function shall be either identical to the return type of the overridden function or covariant with the classes of the functions. If a function D​::​f overrides a function B​::​f, the return types of the functions are covariant if they satisfy the following criteria:

• both are pointers to classes, both are lvalue references to classes, or both are rvalue references to classes
• the class in the return type of B​::​f is the same class as the class in the return type of D​::​f, or is an unambiguous and accessible direct or indirect base class of the class in the return type of D​::​f
• both pointers or references have the same cv-qualification and the class type in the return type of D​::​f has the same cv-qualification as or less cv-qualification than the class type in the return type of B​::​f.