diff --git a/clang/lib/Serialization/ASTWriterDecl.cpp b/clang/lib/Serialization/ASTWriterDecl.cpp
--- a/clang/lib/Serialization/ASTWriterDecl.cpp
+++ b/clang/lib/Serialization/ASTWriterDecl.cpp
@@ -1089,8 +1089,6 @@
     Record.AddStmt(D->getUninstantiatedDefaultArg());
   Code = serialization::DECL_PARM_VAR;
 
-  assert(!D->isARCPseudoStrong()); // can be true of ImplicitParamDecl
-
   // If the assumptions about the DECL_PARM_VAR abbrev are true, use it.  Here
   // we dynamically check for the properties that we optimize for, but don't
   // know are true of all PARM_VAR_DECLs.
diff --git a/clang/test/PCH/externally-retained.m b/clang/test/PCH/externally-retained.m
new file mode 100644
--- /dev/null
+++ b/clang/test/PCH/externally-retained.m
@@ -0,0 +1,30 @@
+// Test for assertion failure due to objc_externally_retained on a function.
+
+// Without PCH
+// RUN: %clang_cc1 -fsyntax-only -verify -fobjc-arc -include %s %s
+
+// With PCH
+// RUN: %clang_cc1 %s -emit-pch -fobjc-arc -o %t
+// RUN: %clang_cc1 -emit-llvm-only -verify %s -fobjc-arc -include-pch %t -debug-info-kind=limited
+
+// expected-no-diagnostics
+
+#ifndef HEADER
+#define HEADER
+//===----------------------------------------------------------------------===//
+// Header
+
+__attribute__((objc_externally_retained)) void doSomething(id someObject);
+
+id sharedObject = 0;
+
+//===----------------------------------------------------------------------===//
+#else
+//===----------------------------------------------------------------------===//
+
+void callDoSomething() {
+  doSomething(sharedObject);
+}
+
+//===----------------------------------------------------------------------===//
+#endif