[clang] [Clang] Partial implementation of support for P3074 (trivial unions) (PR #146815)

[via cfe-commits]

================
@@ -9543,6 +9543,45 @@ bool SpecialMemberDeletionInfo::shouldDeleteForSubobjectCall(
   if (DiagKind == -1)
     return false;
 
+  if (S.Context.getLangOpts().CPlusPlus26 && inUnion() &&
+      CSM == CXXSpecialMemberKind::Destructor) {
+    // [class.dtor]/7 In C++26, a destructor for a union X is only deleted under
+    // the additional conditions that:
+
+    // overload resolution to select a constructor to default-initialize an
+    // object of type X either fails or selects a constructor that is either
+    // deleted or not trivial, or
+    // or X has a variant member V of class type M (or possibly
+    // multi-dimensional array thereof) where V has a default member initializer
+    // and M has a destructor that is non-trivial,
+
+    RecordDecl *Parent = Field->getParent();
+    while (Parent &&
+           (Parent->isAnonymousStructOrUnion() ||
+            (Parent->isUnion() && Parent->getIdentifier() == nullptr))) {
+      if (auto RD = dyn_cast_or_null<RecordDecl>(Parent->getParent()))
+        Parent = RD;
+      else
+        break;
+    }
+
+    auto ParentDecl = dyn_cast<CXXRecordDecl>(Parent);
----------------
Sirraide wrote:

```suggestion
    auto ParentDecl = cast<CXXRecordDecl>(Parent);
```

https://github.com/llvm/llvm-project/pull/146815