Index: lib/AST/RecordLayoutBuilder.cpp
===================================================================
--- lib/AST/RecordLayoutBuilder.cpp
+++ lib/AST/RecordLayoutBuilder.cpp
@@ -2829,15 +2829,14 @@
     CharUnits BaseOffset;
 
     // Respect the external AST source base offset, if present.
-    bool FoundBase = false;
     if (UseExternalLayout) {
-      FoundBase = External.getExternalVBaseOffset(BaseDecl, BaseOffset);
-      if (FoundBase)
-        assert(BaseOffset >= Size && "base offset already allocated");
-    }
-    if (!FoundBase)
+      if (!External.getExternalVBaseOffset(BaseDecl, BaseOffset))
+        BaseOffset = Size;
+    } else
       BaseOffset = Size.alignTo(Info.Alignment);
 
+    assert(BaseOffset >= Size && "base offset already allocated");
+
     VBases.insert(std::make_pair(BaseDecl,
         ASTRecordLayout::VBaseInfo(BaseOffset, HasVtordisp)));
     Size = BaseOffset + BaseLayout.getNonVirtualSize();
Index: test/CodeGenCXX/Inputs/override-layout-packed-base.layout
===================================================================
--- test/CodeGenCXX/Inputs/override-layout-packed-base.layout
+++ test/CodeGenCXX/Inputs/override-layout-packed-base.layout
@@ -3,16 +3,26 @@
 Type: class B<0>
 
 Layout: <ASTRecordLayout
+  Size:40
   FieldOffsets: [0, 32]>
 
 *** Dumping AST Record Layout
 Type: class B<1>
 
 Layout: <ASTRecordLayout
+  Size:40
   FieldOffsets: [0, 32]>
 
 *** Dumping AST Record Layout
 Type: class C
 
 Layout: <ASTRecordLayout
+  Size:88
   FieldOffsets: [80]>
+
+*** Dumping AST Record Layout
+Type: class D
+
+Layout: <ASTRecordLayout
+  Size:120
+  FieldOffsets: [32]>
Index: test/CodeGenCXX/override-layout-packed-base.cpp
===================================================================
--- test/CodeGenCXX/override-layout-packed-base.cpp
+++ test/CodeGenCXX/override-layout-packed-base.cpp
@@ -1,26 +1,40 @@
-// RUN: %clang_cc1 -w -fdump-record-layouts-simple -foverride-record-layout=%S/Inputs/override-layout-packed-base.layout %s | FileCheck %s
+// RUN: %clang_cc1 -triple i686-windows-msvc -w -fdump-record-layouts-simple -foverride-record-layout=%S/Inputs/override-layout-packed-base.layout %s | FileCheck %s
+
+//#pragma pack(push, 1)
 
 // CHECK: Type: class B<0>
+// CHECK:   Size:40
 // CHECK:   FieldOffsets: [0, 32]
 
 // CHECK: Type: class B<1>
+// CHECK:   Size:40
 // CHECK:   FieldOffsets: [0, 32]
 
-//#pragma pack(push, 1)
 template<int I>
 class B {
   int _b1;
   char _b2;
 };
-//#pragma pack(pop)
 
 // CHECK: Type: class C
+// CHECK:   Size:88
 // CHECK:   FieldOffsets: [80]
 
 class C : B<0>, B<1> {
   char _c;
 };
 
+// CHECK: Type: class D
+// CHECK:   Size:120
+// CHECK:   FieldOffsets: [32]
+
+class D : virtual B<0>, virtual B<1> {
+  char _d;
+};
+
+//#pragma pack(pop)
+
 void use_structs() {
   C cs[sizeof(C)];
+  D ds[sizeof(D)];
 }