[llvm-bugs] [Bug 50648] New: Clang should warn on what appear to be buggy friend declarations

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=50648

            Bug ID: 50648
           Summary: Clang should warn on what appear to be buggy friend
                    declarations
           Product: clang
           Version: unspecified
          Hardware: PC
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Frontend
          Assignee: unassignedclangbugs at nondot.org
          Reporter: rnk at google.com
                CC: llvm-bugs at lists.llvm.org, neeilans at live.com,
                    richard-llvm at metafoo.co.uk

Consider code like this:

https://gcc.godbolt.org/z/EEe8xWrzj

class Bar;
namespace foo {
class Foo {
  friend class Bar; // forward declares ::foo::Bar
  Foo();
  int x;
};
}
class Bar {
  Bar() {}
  foo::Foo m; // error
};

It seems like clang should be able to warn and say, "hey, you probably say
`friend class ::Bar;`".

We accept this code in MSVC mode with a warning, so it shouldn't be too hard to
detect when we would do that and provide a warning in that case:
https://reviews.llvm.org/D4443
See `bool FriendSawTagOutsideEnclosingNamespace` in SemaDecl.cpp.

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