Index: cfe/trunk/lib/Sema/SemaDecl.cpp
===================================================================
--- cfe/trunk/lib/Sema/SemaDecl.cpp
+++ cfe/trunk/lib/Sema/SemaDecl.cpp
@@ -8206,6 +8206,9 @@
     }
 
     SemaRef.CheckConversionDeclarator(D, R, SC);
+    if (D.isInvalidType())
+      return nullptr;
+
     IsVirtualOkay = true;
     return CXXConversionDecl::Create(
         SemaRef.Context, cast<CXXRecordDecl>(DC), D.getBeginLoc(), NameInfo, R,
Index: cfe/trunk/test/SemaCXX/PR31422.cpp
===================================================================
--- cfe/trunk/test/SemaCXX/PR31422.cpp
+++ cfe/trunk/test/SemaCXX/PR31422.cpp
@@ -0,0 +1,7 @@
+// RUN: %clang_cc1 -std=c++14 -fsyntax-only -verify %s
+
+// expected-error@+3 {{cannot specify any part of a return type in the declaration of a conversion function; use an alias template to declare a conversion to 'auto (Ts &&...) const'}}
+// expected-error@+2 {{conversion function cannot convert to a function type}}
+struct S {
+  template <typename... Ts> operator auto()(Ts &&... xs) const;
+};