diff --git a/clang/lib/AST/RecordLayoutBuilder.cpp b/clang/lib/AST/RecordLayoutBuilder.cpp
--- a/clang/lib/AST/RecordLayoutBuilder.cpp
+++ b/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.
diff --git a/lldb/test/API/lang/cpp/alignas_base_class/Makefile b/lldb/test/API/lang/cpp/alignas_base_class/Makefile
new file mode 100644
--- /dev/null
+++ b/lldb/test/API/lang/cpp/alignas_base_class/Makefile
@@ -0,0 +1,3 @@
+CXX_SOURCES := main.cpp
+
+include Makefile.rules
diff --git a/lldb/test/API/lang/cpp/alignas_base_class/TestAlignAsBaseClass.py b/lldb/test/API/lang/cpp/alignas_base_class/TestAlignAsBaseClass.py
new file mode 100644
--- /dev/null
+++ b/lldb/test/API/lang/cpp/alignas_base_class/TestAlignAsBaseClass.py
@@ -0,0 +1,16 @@
+import lldb
+from lldbsuite.test.decorators import *
+from lldbsuite.test.lldbtest import *
+from lldbsuite.test import lldbutil
+
+class TestCase(TestBase):
+
+    mydir = TestBase.compute_mydir(__file__)
+
+    @no_debug_info_test
+    def test(self):
+        self.build()
+        self.dbg.CreateTarget(self.getBuildArtifact("a.out"))
+
+        # The offset of f2 should be 8 because of `alignas(8)`.
+        self.expect_expr("(intptr_t)&d3g.f2 - (intptr_t)&d3g", result_value="8")
diff --git a/lldb/test/API/lang/cpp/alignas_base_class/main.cpp b/lldb/test/API/lang/cpp/alignas_base_class/main.cpp
new file mode 100644
--- /dev/null
+++ b/lldb/test/API/lang/cpp/alignas_base_class/main.cpp
@@ -0,0 +1,13 @@
+struct B1 {
+  char f1;
+};
+
+struct alignas(8) B2 {
+  char f2;
+};
+
+struct D : B1, B2 {};
+
+D d3g;
+
+int main() {}