Fix PR18806: Canonicalize the replacement type when deserializing a SubstTemplateTypeParmType

What's going on in the test case (without the patch applied) is this:

When the header is parsed, decltype(B()) is canonicalized to decltype(Y()),
because that was the first parsed equivalent decltype expression. Hence, the
TemplateSpecializationType for Id<decltype(B())> ends up with
SubstTemplateTypeParmType(T, decltype(Y())) as the AliasedType member.

When the PCH file is included and the AST reader reads Id<decltype(B())>, it
sees decltype(B()) before decltype(Y()). So, this time decltype(B()) ends up
being the canonical type for both decltypes, which leads to an assert violation
when the reader calls getSubstTemplateTypeParmType with the non-canonical
decltype(Y()) as the replacement type.

Reviewers: rsmith

Reviewed By: rsmith

CC: cfe-commits, aemerson

Differential Revision: http://llvm-reviews.chandlerc.com/D3073

llvm-svn: 204005

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: {

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