Strange behavior in GCC: rvalue ref and lvalue ref are covariant return types

Question

I have this little piece of code:

struct Res { };

struct A {
    virtual Res &&foo();
};

struct B : A {
    Res &foo() override;
};

It compiles in GCC, but not in Clang: https://godbolt.org/z/65rffW .

According to the standard language quoted here, the lvalue reference is not a covariant return type for an rvalue reference.

Why doesn't GCC issue an error?

Answer 1

This was actually the subject of a standard defect report, a quite old one:

960. Covariant functions and lvalue/rvalue references Section: 11.7.3 [class.virtual] Status: CD2 Submitter: James Widman Date: 1 September, 2009

[Voted into WP at March, 2010 meeting.]

11.7.3 [class.virtual] paragraph 5 requires that covariant return types be either both pointers or both references, but it does not specify that references must be both lvalue references or both rvalue references. Presumably this is an oversight.

Proposed resolution (February, 2010):

Change 11.7.3 [class.virtual] paragraph 5 bullet 1 as follows:

...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 classes¹⁰⁶

...

From http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#960

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The code is undoubtedly ill-formed, Clang and MSVC (at least) did address the issue whereas GCC did not.

I filed a bug report: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99664

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Update:

The bug was fixed, test case here. Will be included in gcc 12.