[llvm-bugs] [Bug 34630] New: __builtin_offsetof doesn't allow scope qualifiers

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=34630

            Bug ID: 34630
           Summary: __builtin_offsetof doesn't allow scope qualifiers
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: C++
          Assignee: unassignedclangbugs at nondot.org
          Reporter: myriachan at gmail.com
                CC: dgregor at apple.com, llvm-bugs at lists.llvm.org

The following code does not compile, with a syntax error:

#include <cstddef>

struct A { int x; };
struct B { };
struct C : A, B { };

int meow()
{
  return offsetof(C, A::x);
}

error: expected ')'

Whether this should be allowed in C++ is an interesting question.  C++ defers
to C, which says a "member-designator" here, but then doesn't define that. 
(There's a WG14 defect report somewhere on this, to be fixed for C2x?)

Normally, this wouldn't be necessary, because the first parameter should be a
standard-layout type, which can't have non-static members of conflicting names.
 But pretty much every compiler supports offsetof() on everything but virtual
inheritance cases, so people use it.  (Also, there's a C++ proposal in the
works to formally allow offsetof for any class not involving the word
"virtual".)

GCC allows this.

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