Can abstract types be used as the return type of a pure virtual function?

Question

If I understand correctly, abstract types cannot be used as the return type of a pure virtual function. However, if a trailing return type is used, then gcc (but not clang) accepts the following code:

struct S {
    virtual auto f() -> S = 0; 
};

demo.

Is this a gcc bug, or does the language not require a diagnostic to be issued for this code?

Answer 1

According to C++17 [class.abstract]/3, an abstract type cannot be used as the return type of any function:

An abstract class shall not be used as a parameter type, as a function return type, or as the type of an explicit conversion. Pointers and references to an abstract class can be declared. [ Example:

shape x;           // error: object of abstract class
shape* p;          // OK
shape f();         // error
void g(shape);     // error
shape& h(shape&);  // OK

— end example ]

There doesn't seem to be any special exception for pure virtual functions. Therefore, it seems that the answer is that the compiler is required to issue a diagnostic for your code.