[llvm-bugs] [Bug 47843] New: clang accepts an abstract class in function return type

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=47843

            Bug ID: 47843
           Summary: clang accepts an abstract class in function return
                    type
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: C++
          Assignee: unassignedclangbugs at nondot.org
          Reporter: tangyixuan at mail.dlut.edu.cn
                CC: blitzrakete at gmail.com, dgregor at apple.com,
                    erik.pilkington at gmail.com, llvm-bugs at lists.llvm.org,
                    richard-llvm at metafoo.co.uk

Hi, the following code maybe is ill-formed, since the abstract class shall not
be used as a function return type in most cases. Clang accepts it without error
diagnostics.

$ cat s.cpp

template<typename T>
struct S{};

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

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

$ clang++ -c s.cpp
<success>

$ 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;
      |

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