Index: clang/lib/AST/RecordLayoutBuilder.cpp
===================================================================
--- clang/lib/AST/RecordLayoutBuilder.cpp
+++ clang/lib/AST/RecordLayoutBuilder.cpp
@@ -1187,11 +1187,10 @@
   // Query the external layout to see if it provides an offset.
   bool HasExternalLayout = false;
   if (UseExternalLayout) {
-    // FIXME: This appears to be reversed.
     if (Base->IsVirtual)
-      HasExternalLayout = External.getExternalNVBaseOffset(Base->Class, Offset);
-    else
       HasExternalLayout = External.getExternalVBaseOffset(Base->Class, Offset);
+    else
+      HasExternalLayout = External.getExternalNVBaseOffset(Base->Class, Offset);
   }
 
   // Clang <= 6 incorrectly applied the 'packed' attribute to base classes.
Index: lldb/test/Shell/Expr/Inputs/layout.cpp
===================================================================
--- /dev/null
+++ lldb/test/Shell/Expr/Inputs/layout.cpp
@@ -0,0 +1,30 @@
+#include <cstdint>
+
+struct B1 {
+  uint8_t a;
+};
+
+struct D1 : public B1 {
+  uint8_t a;
+  uint32_t ID;
+  uint8_t b;
+};
+
+struct Mixin : public D1 {
+  uint8_t a;
+  uint32_t *arr[3];
+};
+
+struct B2 {
+  uint32_t a;
+};
+
+class D2 : public B2, public Mixin {};
+
+D2 sg;
+
+int main() {
+  D2 s;
+
+  return s.ID;
+}
Index: lldb/test/Shell/Expr/TestLayoutNonVirtualBaseClasses.test
===================================================================
--- /dev/null
+++ lldb/test/Shell/Expr/TestLayoutNonVirtualBaseClasses.test
@@ -0,0 +1,7 @@
+# RUN: %clangxx_host %p/Inputs/layout.cpp -g -o %t
+
+# RUN: %lldb %t -b -s %s | FileCheck %s
+
+expr (intptr_t)&sg.ID - (intptr_t)&sg
+# CHECK: (lldb) expr (intptr_t)&sg.ID - (intptr_t)&sg
+# CHECK: (long) $0 = 12