[clang] Fix to msvc::no_unique_address causing assert when used with __declspec(empty_bases) (PR #74776)

[Amy Huang via cfe-commits]

https://github.com/amykhuang created https://github.com/llvm/llvm-project/pull/74776

no_unique_address makes it possible for a class to be empty and have non-zero virtual size, so just remove this assert.

Bug: https://github.com/llvm/llvm-project/issues/74442

>From c3dedfd535f037fb54a8e04640f9332e655a9a7d Mon Sep 17 00:00:00 2001
From: Amy Huang <akhuang at google.com>
Date: Thu, 7 Dec 2023 14:08:24 -0800
Subject: [PATCH] Fix to msvc::no_unique_address not interacting well with
 __declspec(empty_bases)

---
 clang/lib/AST/RecordLayoutBuilder.cpp         |  4 +-
 .../Layout/ms-x86-declspec-empty_bases.cpp    | 60 +++++++++++++++++++
 2 files changed, 62 insertions(+), 2 deletions(-)

diff --git a/clang/lib/AST/RecordLayoutBuilder.cpp b/clang/lib/AST/RecordLayoutBuilder.cpp
index f1f2275da44dc..73b248ffe5899 100644
--- a/clang/lib/AST/RecordLayoutBuilder.cpp
+++ b/clang/lib/AST/RecordLayoutBuilder.cpp
@@ -2936,8 +2936,8 @@ void MicrosoftRecordLayoutBuilder::layoutNonVirtualBase(
   }
 
   if (!FoundBase) {
-    if (MDCUsesEBO && BaseDecl->isEmpty()) {
-      assert(BaseLayout.getNonVirtualSize() == CharUnits::Zero());
+    if (MDCUsesEBO && BaseDecl->isEmpty() &&
+        (BaseLayout.getNonVirtualSize() == CharUnits::Zero())) {
       BaseOffset = CharUnits::Zero();
     } else {
       // Otherwise, lay the base out at the end of the MDC.
diff --git a/clang/test/Layout/ms-x86-declspec-empty_bases.cpp b/clang/test/Layout/ms-x86-declspec-empty_bases.cpp
index cc13a980cb5db..4738ce5720f75 100644
--- a/clang/test/Layout/ms-x86-declspec-empty_bases.cpp
+++ b/clang/test/Layout/ms-x86-declspec-empty_bases.cpp
@@ -264,3 +264,63 @@ int _ = sizeof(G);
 // CHECK-NEXT:            | [sizeof=12, align=4,
 // CHECK-NEXT:            |  nvsize=12, nvalign=4]
 }
+
+namespace test5 {
+
+struct A {
+  int a;
+};
+struct B {
+  int b;
+};
+struct C {};
+struct __declspec(align(16)) D {};
+struct E {
+  [[msvc::no_unique_address]] C c;
+};
+struct __declspec(empty_bases) X : A, D, B, C, E {
+};
+
+// CHECK: *** Dumping AST Record Layout
+// CHECK-NEXT:          0 | struct test5::A
+// CHECK-NEXT:          0 |   int a
+// CHECK-NEXT:            | [sizeof=4, align=4,
+// CHECK-NEXT:            |  nvsize=4, nvalign=4]
+
+// CHECK: *** Dumping AST Record Layout
+// CHECK-NEXT:          0 | struct test5::D (empty)
+// CHECK-NEXT:            | [sizeof=16, align=16,
+// CHECK-NEXT:            |  nvsize=0, nvalign=16]
+
+// CHECK: *** Dumping AST Record Layout
+// CHECK-NEXT:          0 | struct test5::B
+// CHECK-NEXT:          0 |   int b
+// CHECK-NEXT:            | [sizeof=4, align=4,
+// CHECK-NEXT:            |  nvsize=4, nvalign=4]
+
+// CHECK: *** Dumping AST Record Layout
+// CHECK-NEXT:          0 | struct test5::C (empty)
+// CHECK-NEXT:            | [sizeof=1, align=1,
+// CHECK-NEXT:            |  nvsize=0, nvalign=1]
+
+// CHECK: *** Dumping AST Record Layout
+// CHECK-NEXT:          0 | struct test5::E (empty)
+// CHECK-NEXT:          0 |   struct test5::C c (empty)
+// CHECK-NEXT:            | [sizeof=1, align=1,
+// CHECK-NEXT:            |  nvsize=1, nvalign=1]
+
+// CHECK: *** Dumping AST Record Layout
+// CHECK-NEXT:          0 | struct test5::X
+// CHECK-NEXT:          0 |   struct test5::A (base)
+// CHECK-NEXT:          0 |     int a
+// CHECK-NEXT:          0 |   struct test5::D (base) (empty)
+// CHECK-NEXT:          0 |   struct test5::C (base) (empty)
+// CHECK-NEXT:          4 |   struct test5::B (base)
+// CHECK-NEXT:          4 |     int b
+// CHECK-NEXT:          8 |   struct test5::E (base) (empty)
+// CHECK-NEXT:          8 |     struct test5::C c (empty)
+// CHECK-NEXT:            | [sizeof=16, align=16,
+// CHECK-NEXT:            |  nvsize=16, nvalign=16]
+
+int _ = sizeof(X);
+}