Index: clang/lib/Sema/SemaExprCXX.cpp
===================================================================
--- clang/lib/Sema/SemaExprCXX.cpp
+++ clang/lib/Sema/SemaExprCXX.cpp
@@ -5118,8 +5118,15 @@
     assert(Self.Context.hasSameUnqualifiedType(LhsT, RhsT)
              == (lhsRecord == rhsRecord));
 
+    // Unions are never base classes, and never have base classes.
+    // It doesn't matter if they are complete or not. See PR#41843
+    if (lhsRecord && lhsRecord->getDecl()->isUnion())
+      return false;
+    if (rhsRecord && rhsRecord->getDecl()->isUnion())
+      return false;
+
     if (lhsRecord == rhsRecord)
-      return !lhsRecord->getDecl()->isUnion();
+      return true;
 
     // C++0x [meta.rel]p2:
     //   If Base and Derived are class types and are different types
Index: clang/test/SemaCXX/type-traits.cpp
===================================================================
--- clang/test/SemaCXX/type-traits.cpp
+++ clang/test/SemaCXX/type-traits.cpp
@@ -14,6 +14,7 @@
 enum Enum { EV };
 struct POD { Enum e; int i; float f; NonPOD* p; };
 struct Empty {};
+struct IncompleteStruct;
 typedef Empty EmptyAr[10];
 typedef Empty EmptyArNB[];
 typedef Empty EmptyArMB[1][2];
@@ -1915,6 +1916,20 @@
   { int arr[F(__is_base_of(Base, NonderivedTemp<int>))]; }
   { int arr[F(__is_base_of(Base, UndefinedTemp<int>))]; } // expected-error {{implicit instantiation of undefined template 'UndefinedTemp<int>'}}
 
+  { int arr[F(__is_base_of(IncompleteUnion, IncompleteUnion))]; }
+  { int arr[F(__is_base_of(Union, IncompleteUnion))]; }
+  { int arr[F(__is_base_of(IncompleteUnion, Union))]; }
+  { int arr[F(__is_base_of(IncompleteStruct, IncompleteUnion))]; }
+  { int arr[F(__is_base_of(IncompleteUnion, IncompleteStruct))]; }
+  { int arr[F(__is_base_of(Empty, IncompleteUnion))]; }
+  { int arr[F(__is_base_of(IncompleteUnion, Empty))]; }
+  { int arr[F(__is_base_of(int, IncompleteUnion))]; }
+  { int arr[F(__is_base_of(IncompleteUnion, int))]; }
+  { int arr[F(__is_base_of(Empty, Union))]; }
+  { int arr[F(__is_base_of(Union, Empty))]; }
+  { int arr[F(__is_base_of(int, Empty))]; }
+  { int arr[F(__is_base_of(Union, int))]; }
+
   isBaseOfT<Base, Derived>();
   isBaseOfF<Derived, Base>();