Index: llvm/trunk/lib/TableGen/Record.cpp
===================================================================
--- llvm/trunk/lib/TableGen/Record.cpp
+++ llvm/trunk/lib/TableGen/Record.cpp
@@ -141,10 +141,6 @@
   if (RTy->getRecord() == Rec || Rec->isSubClassOf(RTy->getRecord()))
     return true;
 
-  for (const auto &SCPair : RTy->getRecord()->getSuperClasses())
-    if (Rec->isSubClassOf(SCPair.first))
-      return true;
-
   return false;
 }
 
@@ -173,6 +169,16 @@
         return NewType2;
     }
   }
+
+  if (ListRecTy *ListTy1 = dyn_cast<ListRecTy>(T1)) {
+    if (ListRecTy *ListTy2 = dyn_cast<ListRecTy>(T2)) {
+      RecTy* NewType = resolveTypes(ListTy1->getElementType(),
+                                    ListTy2->getElementType());
+      if (NewType)
+        return NewType->getListTy();
+    }
+  }
+
   return nullptr;
 }
 
Index: llvm/trunk/lib/TableGen/TGParser.cpp
===================================================================
--- llvm/trunk/lib/TableGen/TGParser.cpp
+++ llvm/trunk/lib/TableGen/TGParser.cpp
@@ -198,6 +198,8 @@
 
   // Since everything went well, we can now set the "superclass" list for the
   // current record.
+  CurRec->addSuperClass(SC, SubClass.RefRange);
+
   ArrayRef<std::pair<Record *, SMRange>> SCs = SC->getSuperClasses();
   for (const auto &SCPair : SCs) {
     if (CurRec->isSubClassOf(SCPair.first))
@@ -205,11 +207,6 @@
                    "Already subclass of '" + SCPair.first->getName() + "'!\n");
     CurRec->addSuperClass(SCPair.first, SCPair.second);
   }
-
-  if (CurRec->isSubClassOf(SC))
-    return Error(SubClass.RefRange.Start,
-                 "Already subclass of '" + SC->getName() + "'!\n");
-  CurRec->addSuperClass(SC, SubClass.RefRange);
   return false;
 }
 
@@ -1067,12 +1064,10 @@
         return nullptr;
       }
 
-      if (MHSTy->typeIsConvertibleTo(RHSTy)) {
-        Type = RHSTy;
-      } else if (RHSTy->typeIsConvertibleTo(MHSTy)) {
-        Type = MHSTy;
-      } else {
-        TokError("inconsistent types for !if");
+      Type = resolveTypes(MHSTy, RHSTy);
+      if (!Type) {
+        TokError(Twine("inconsistent types '") + MHSTy->getAsString() +
+                 "' and '" + RHSTy->getAsString() + "' for !if");
         return nullptr;
       }
       break;
Index: llvm/trunk/test/TableGen/if-type.td
===================================================================
--- llvm/trunk/test/TableGen/if-type.td
+++ llvm/trunk/test/TableGen/if-type.td
@@ -0,0 +1,11 @@
+// RUN: not llvm-tblgen %s 2>&1 | FileCheck %s
+// XFAIL: vg_leak
+
+class A<int dummy> {}
+class B<int dummy> : A<dummy> {}
+class C<int dummy> : A<dummy> {}
+
+// CHECK: Value 'x' of type 'C' is incompatible with initializer '{{.*}}' of type 'A'
+class X<int cc, B b, C c> {
+  C x = !if(cc, b, c);
+}
Index: llvm/trunk/test/TableGen/if.td
===================================================================
--- llvm/trunk/test/TableGen/if.td
+++ llvm/trunk/test/TableGen/if.td
@@ -69,10 +69,34 @@
 // CHECK-NEXT: int i = 0;
 def DI1: I1<1>;
 
-// CHECK: def DI2 {     // I1 I2
+// CHECK: def DI2 {
 // CHECK-NEXT: int i = 0;
 def DI2: I2<1>;
 
+// Check that !if with operands of different subtypes can initialize a
+// supertype variable.
+//
+// CHECK: def EXd1 {
+// CHECK:   E x = E1d;
+// CHECK: }
+//
+// CHECK: def EXd2 {
+// CHECK:   E x = E2d;
+// CHECK: }
+class E<int dummy> {}
+class E1<int dummy> : E<dummy> {}
+class E2<int dummy> : E<dummy> {}
+
+class EX<int cc, E1 b, E2 c> {
+  E x = !if(cc, b, c);
+}
+
+def E1d : E1<0>;
+def E2d : E2<0>;
+
+def EXd1 : EX<1, E1d, E2d>;
+def EXd2 : EX<0, E1d, E2d>;
+
 // CHECK:      def One
 // CHECK-NEXT: list<int> first = [1, 2, 3];
 // CHECK-NEXT: list<int> rest = [1, 2, 3];