Index: clang/lib/Sema/SemaDecl.cpp
===================================================================
--- clang/lib/Sema/SemaDecl.cpp
+++ clang/lib/Sema/SemaDecl.cpp
@@ -8066,6 +8066,9 @@
     }
 
     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: clang/test/Sema/conversion_function_to_function.cpp
===================================================================
--- /dev/null
+++ clang/test/Sema/conversion_function_to_function.cpp
@@ -0,0 +1,9 @@
+// RUN: not %clang_cc1 -fsyntax-only -std=c++14 %s 2>&1 | FileCheck %s
+
+struct S {
+  template <typename... Ts> operator auto()(Ts &&... xs) const;
+};
+
+// CHECK: error: 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'
+// CHECK: error: conversion function cannot convert to a function type
+// CHECK-NOT: Assertion{{.*}}failed