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

[Timm Baeder via cfe-commits]

https://github.com/tbaederr updated https://github.com/llvm/llvm-project/pull/154342

>From 71038d3946d2baa9b4b45d3225d05a6bfc6f0d71 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Timm=20B=C3=A4der?= <tbaeder at redhat.com>
Date: Tue, 19 Aug 2025 15:55:37 +0200
Subject: [PATCH] [clang][bytecode] Fix comparing pointers to union members

If one of them is a one-past-end pointer.
---
 clang/lib/AST/ByteCode/Pointer.cpp |  2 ++
 clang/test/AST/ByteCode/unions.cpp | 12 ++++++++++++
 2 files changed, 14 insertions(+)

diff --git a/clang/lib/AST/ByteCode/Pointer.cpp b/clang/lib/AST/ByteCode/Pointer.cpp
index 7c6eb74da205c..89d9829399302 100644
--- a/clang/lib/AST/ByteCode/Pointer.cpp
+++ b/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;
diff --git a/clang/test/AST/ByteCode/unions.cpp b/clang/test/AST/ByteCode/unions.cpp
index 8ed76258b879a..6bccbda23f468 100644
--- a/clang/test/AST/ByteCode/unions.cpp
+++ b/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