[Bug c++/97443] New: gcc rejects an abstract class that could be used as a function return type due to the new rules

[tangyixuan at mail dot dlut.edu.cn]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97443

            Bug ID: 97443
           Summary: gcc rejects an abstract class that could be used as a
                    function return type due to the new rules
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: tangyixuan at mail dot dlut.edu.cn
  Target Milestone: ---

Hi, the abstract class could be used as a function return type, as well as the
parameter type and the type of an explicit conversion. Please see:
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0929r2.html

gcc rejects the following valid program:

$ cat s.cpp

template<typename T>
struct S{};

struct A
{
  virtual void func() = 0;
};

int main()
{
  S<A(int)> s;
}

$ g++ -c s.cpp
s.cpp: In function ‘int main()’:
s.cpp:11:5: error: invalid abstract return type ‘A’
   23 |   S<A(int)> s;
      |     ^
s.cpp:4:8: note:   because the following virtual functions are pure within ‘A’:
   16 | struct A
      |        ^
s.cpp:6:16: note:     ‘virtual void A::f()’
   18 |   virtual void f() = 0;
      |