[llvm-bugs] [Bug 40809] New: Zero-width bit-field in an anonymous subobject

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=40809

            Bug ID: 40809
           Summary: Zero-width bit-field in an anonymous subobject
           Product: clang
           Version: unspecified
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: -New Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: aaron at aaronballman.com
                CC: htmldeveloper at gmail.com, llvm-bugs at lists.llvm.org,
                    neeilans at live.com, richard-llvm at metafoo.co.uk

I believe the following structure has the incorrect layout according to the C
standard:

struct S {
  int i : 1;
  union {
    int : 0;
  };
  int j : 1;
};

int main(void) {
  return sizeof(struct S) == sizeof(int);
}

main() should return 0, but in Clang it returns 1.

C17 6.7.2.1p13 makes it clear that the members of the anonymous union are
members of the enclosing struct, so the zero-width bit-field should introduce
an allocation boundary via p12.

FWIW, Clang matches GCC's behavior here, but not MSVC's behavior.

I believe this is an instance where the behavior in C and C++ have diverged,
because [class.union.anon]p1 (http://eel.is/c++draft/class.union.anon#1) says
that the anonymous union members are only members of the containing object for
purposes of name lookup.

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