Index: lib/Serialization/ASTReaderDecl.cpp
===================================================================
--- lib/Serialization/ASTReaderDecl.cpp
+++ lib/Serialization/ASTReaderDecl.cpp
@@ -3000,6 +3000,8 @@
 
   for (unsigned I = 0, N = FromTP->size(); I != N; ++I) {
     NamedDecl *FromParam = FromTP->getParam(N - I - 1);
+    if (FromParam->isParameterPack())
+      continue;
     NamedDecl *ToParam = ToTP->getParam(N - I - 1);
 
     if (auto *FTTP = dyn_cast<TemplateTypeParmDecl>(FromParam)) {
Index: test/PCH/cxx-variadic-templates-with-default-params.h
===================================================================
--- test/PCH/cxx-variadic-templates-with-default-params.h
+++ test/PCH/cxx-variadic-templates-with-default-params.h
@@ -0,0 +1,8 @@
+// PR25271: Ensure that variadic template parameter defaults PCH roundtrip
+template<unsigned T=123, unsigned... U>
+class dummy;
+
+template<unsigned T, unsigned... U>
+class dummy {
+    int field[T];
+};
Index: test/PCH/cxx-variadic-templates-with-default-params.cpp
===================================================================
--- test/PCH/cxx-variadic-templates-with-default-params.cpp
+++ test/PCH/cxx-variadic-templates-with-default-params.cpp
@@ -0,0 +1,12 @@
+// Test this without pch.
+// RUN: %clang_cc1 -std=c++11 -include %S/cxx-variadic-templates-with-default-params.h -fsyntax-only -verify %s
+
+// Test with pch.
+// RUN: %clang_cc1 -std=c++11 -x c++-header -emit-pch -o %t %S/cxx-variadic-templates-with-default-params.h
+// RUN: %clang_cc1 -std=c++11 -include-pch %t -fsyntax-only -verify %s
+
+// expected-no-diagnostics
+void f() {
+    dummy<> x;
+    (void)x;
+}