Regression(r274291): Missing diagnostic for return of `std::unique_ptr<Derived>` in C++11

[llvmbot]

Bugzilla Link	31461
Version	trunk
OS	Linux
Reporter	LLVM Bugzilla Contributor
CC	@DougGregor,@nico,@zygoloid

Extended Description

Consider the following

#include <memory>

struct A {};
struct B : public A {};

std::unique_ptr<A> f() {
  auto result = std::unique_ptr<B>(new B);
  return result;
}

In C++11, the return line has to have an std::move, since the type of |result| doesn't match the return type of the function.

However, it compiles with current clang++

Expected:

$ clang++ -std=c++11 -c test.cpp
test.cpp:8:10: error: no viable conversion from 'unique_ptr<struct B, default_delete<struct B>>' to 'unique_ptr<struct A, default_delete<struct A>>'
  return result;
         ^~~~~~
...
$ clang++ --version
clang version 3.9.0 (trunk 273760)
...

Actual:

$ clang++ -std=c++11 -c test.cpp
$ clang++ --version
clang version 3.9.0 (trunk 274367)
...

Since C++14 does allow this to compile, I suspect that http://reviews.llvm.org/D21619 inadvertently caused this.

[nico]

Erik, was this an intentional change?

[llvmbot]

Erik, was this an intentional change?

Yes, this change originated as a defect report (DR1579), and our policy for defect reports is to back port it to older standards. I suppose it would be nice to issue an extension warning when compiling in c++11 mode, but the compiler behavior is intended.

[frederick-vs-ja]

See also WG21 P2413 (which isn't adopted yet). If the paper is adopted, I think it should be treated as a DR and applied to old modes, and the posted example should be rejected. CC @leni536.

Maybe there should be some warning for things similar to the current unique_ptr, though.