[PATCH] Fix to PR5683 - issue diagnostic for pointer subtraction with type of size zero.

[Richard Smith]

================
Comment at: lib/AST/ExprConstant.cpp:6564-6570
@@ -6563,1 +6563,9 @@
 
+        // As an extension, a type may have zero size (empty struct or union in
+        // C, array of zero length).  Meaning of pointer difference in such
+        // case is unspecified.
+        if (ElementSize.isZero()) {
+          Result = APValue(Info.Ctx.MakeIntValue(0, E->getType()));
+          return true;
+        }
+
----------------
Should we really treat the result as a constant zero in this case? GCC treats it as non-constant (and emits a divide-by-zero at runtime.) C++11 requires us to treat expressions with undefined behavior as non-constant.


http://llvm-reviews.chandlerc.com/D637