diff --git a/clang/lib/Sema/SemaDeclCXX.cpp b/clang/lib/Sema/SemaDeclCXX.cpp
--- a/clang/lib/Sema/SemaDeclCXX.cpp
+++ b/clang/lib/Sema/SemaDeclCXX.cpp
@@ -2729,6 +2729,10 @@
       KnownBase = Bases[idx];
       Bases[NumGoodBases++] = Bases[idx];
 
+      if (NewBaseType->isIncompleteType()) {
+        assert(NewBaseType->isDependentType());
+        continue;
+      }
       // Note this base's direct & indirect bases, if there could be ambiguity.
       if (Bases.size() > 1)
         NoteIndirectBases(Context, IndirectBaseTypes, NewBaseType);
diff --git a/clang/test/SemaCXX/base-class-ambiguity-check.cpp b/clang/test/SemaCXX/base-class-ambiguity-check.cpp
--- a/clang/test/SemaCXX/base-class-ambiguity-check.cpp
+++ b/clang/test/SemaCXX/base-class-ambiguity-check.cpp
@@ -7,3 +7,11 @@
   // Test that this code no longer causes a crash in Sema. rdar://23291875
   struct Derived : Base, T {};
 };
+
+
+template <typename T> class Foo2 {
+  struct Base;
+
+  // Should not crash on an incomplete-type base specifier.
+  struct Derived : Base {};
+};
diff --git a/clang/test/SemaCXX/ms-interface.cpp b/clang/test/SemaCXX/ms-interface.cpp
--- a/clang/test/SemaCXX/ms-interface.cpp
+++ b/clang/test/SemaCXX/ms-interface.cpp
@@ -106,3 +106,10 @@
 static_assert(!__is_interface_class(IUnknown), "oops");
 static_assert(!__is_interface_class(IFaceStruct), "oops");
 static_assert(!__is_interface_class(IFaceInheritsStruct), "oops");
+
+template<typename>
+class TemplateContext {
+  class Base;
+  // Should not crash on an incomplete-type base specifier.
+  __interface Foo : Base {};
+};