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?
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.