Index: Sema/SemaDeclCXX.cpp
===================================================================
--- Sema/SemaDeclCXX.cpp
+++ Sema/SemaDeclCXX.cpp
@@ -2417,9 +2417,16 @@
   // Attach the remaining base class specifiers to the derived class.
   Class->setBases(Bases.data(), NumGoodBases);
 
+  // Check that the only base classes that are duplicate are virtual.
   for (unsigned idx = 0; idx < NumGoodBases; ++idx) {
     // Check whether this direct base is inaccessible due to ambiguity.
     QualType BaseType = Bases[idx]->getType();
+
+    // Skip all dependent types in templates being used as base specifiers.
+    // Checks below assume that the base specifier is a CXXRecord.
+    if (BaseType->isDependentType())
+      continue;
+
     CanQualType CanonicalBase = Context.getCanonicalType(BaseType)
       .getUnqualifiedType();
 
Index: SemaCXX/base-class-ambiguity-check.cpp
===================================================================
--- /dev/null
+++ SemaCXX/base-class-ambiguity-check.cpp
@@ -0,0 +1,8 @@
+// RUN: %clang_cc1 -fsyntax-only %s
+
+template <typename T> class Foo {
+  struct Base : T {};
+
+  // Up to commit 680b3a8619 (2018-01-10) this produced a crash in Sema.
+  struct Derived : Base, T {};
+};