Index: lib/Sema/SemaExpr.cpp
===================================================================
--- lib/Sema/SemaExpr.cpp
+++ lib/Sema/SemaExpr.cpp
@@ -14044,6 +14044,9 @@
   if (ReturnType->isVoidType() || !ReturnType->isIncompleteType())
     return false;
 
+  if (Loc.isInvalid())
+    return true;
+
   // If we're inside a decltype's expression, don't check for a valid return
   // type or construct temporaries until we know whether this is the last call.
   if (ExprEvalContexts.back().IsDecltype) {
Index: test/Sema/PR25668.cpp
===================================================================
--- /dev/null
+++ test/Sema/PR25668.cpp
@@ -0,0 +1,9 @@
+// RUN: %clang_cc1 -verify -fsyntax-only -std=c++11 %s
+// expected-no-assertion-crash
+
+template <class> class A {};
+struct B {
+  A<int> operator[](int);
+} mask_ {
+  mask[2]; // expected-error{{use of undeclared identifier 'mask'}}
+};