2 files changed, 34 insertions, 3 deletions
diff --git a/clang/lib/Serialization/ASTReader.cpp b/clang/lib/Serialization/ASTReader.cpp
index 7781a7d1b05..a0ee3d37c5c 100644
--- a/clang/lib/Serialization/ASTReader.cpp
+++ b/clang/lib/Serialization/ASTReader.cpp
@@ -5203,9 +5203,9 @@ QualType ASTReader::readTypeRecord(unsigned Index) {
     unsigned Idx = 0;
     QualType Parm = readType(*Loc.F, Record, Idx);
     QualType Replacement = readType(*Loc.F, Record, Idx);
-    return
-      Context.getSubstTemplateTypeParmType(cast<TemplateTypeParmType>(Parm),
-                                            Replacement);
+    return Context.getSubstTemplateTypeParmType(
+        cast<TemplateTypeParmType>(Parm),
+        Context.getCanonicalType(Replacement));
   }
 
   case TYPE_SUBST_TEMPLATE_TYPE_PARM_PACK: {
diff --git a/clang/test/PCH/pr18806.cpp b/clang/test/PCH/pr18806.cpp
new file mode 100644
index 00000000000..c28320db956
--- /dev/null
+++ b/clang/test/PCH/pr18806.cpp
@@ -0,0 +1,31 @@
+// RUN: %clang_cc1 -std=c++11 -emit-pch -o %t %s
+// RUN: %clang_cc1 -std=c++11 -include-pch %t -verify %s
+
+// expected-no-diagnostics
+
+// Before the patch, this test triggered an assert violation in
+// ASTContext::getSubstTemplateTypeParmType.
+
+#ifndef HEADER_INCLUDED
+#define HEADER_INCLUDED
+
+template <typename T>
+using Id = T;
+
+template <typename X>
+struct Class1 {
+  template <typename Y, typename = decltype(Y())>
+  struct Nested1;
+};
+
+template <typename A>
+struct Class2 {
+  template <typename B, typename = Id<decltype(B())>>
+  struct Nested2;
+};
+
+#else
+
+Class2<char> test;
+
+#endif