[clang][bytecode] Fix comparing pointers to union members (llvm#154342)

If one of them is a one-past-end pointer.

Author: tbaederr

clang/lib/AST/ByteCode/Pointer.cpp

@@ -375,6 +375,8 @@ size_t Pointer::computeOffsetForComparison() const {
     }
 
     if (const Record *R = P.getBase().getRecord(); R && R->isUnion()) {
+      if (P.isOnePastEnd())
+        ++Result;
       // Direct child of a union - all have offset 0.
       P = P.getBase();
       continue;

clang/test/AST/ByteCode/unions.cpp

@@ -966,3 +966,15 @@ namespace AddressComparison {
   static_assert(&U2.a[0] != &U2.b[1]);
   static_assert(&U2.a[0] == &U2.b[1]); // both-error {{failed}}
 }
+
+#if __cplusplus >= 202002L
+namespace UnionMemberOnePastEnd {
+  constexpr bool b() {
+    union  {
+      int p;
+    };
+    return &p == (&p + 1);
+  }
+  static_assert(!b());
+}
+#endif