[dcl.fct.def.general] p2

The type of a parameter or the return type for a function definition shall not be a (possibly cv-qualified) class type that is incomplete or abstract within the function body unless the function is deleted ([dcl.fct.def.delete]).

Consider these examples

Example 1:

struct C;
template<class T>
struct D{
    auto show(){
        return reinterpret_cast<C&&>((int&&)0);  // ignore the UB here
    }
};

Example 2:

struct C;
struct D{
    template<class T>
    T show(){
        return reinterpret_cast<C&&>((int&&)0);  // ignore the UB here
    }
};

Example 3:

struct C;
struct D{
    template<class T>
    C show(){
        return T{};
    }
};

Example 4:

struct C;
struct D{
    auto show(){
        return reinterpret_cast<C&&>((int&&)0);  // ignore the UB here
    }
};

The first two examples are accepted by both GCC and Clang. Conversely, the last two are rejected by both of them. It seems [dcl.fct.def.general] p2 didn't cover the templated entity case where the return type or parameter types are placeholder types or dependent types. The modification is that:

If the function definition is not for a templated function:

• the type of a parameter or the return type(possibly deduced from placeholder type deduction) shall not be a (possibly cv-qualified) class type that is incomplete or abstract within the function body unless the function is deleted.

Otherwise

if the type is a placeholder type or a dependent type:

• the type shall not be a (possibly cv-qualified) class type that is incomplete or abstract at the point of instantiation unless the function is deleted.

Otherwise

• the type shall not be a (possibly cv-qualified) class type that is incomplete or abstract within the function body at the definition context unless the function is deleted.