Index: lib/Serialization/ASTReader.cpp
===================================================================
--- lib/Serialization/ASTReader.cpp
+++ lib/Serialization/ASTReader.cpp
@@ -50,6 +50,7 @@
 #include "llvm/Support/FileSystem.h"
 #include "llvm/Support/MemoryBuffer.h"
 #include "llvm/Support/Path.h"
+#include "llvm/Support/PrettyStackTrace.h"
 #include "llvm/Support/SaveAndRestore.h"
 #include "llvm/Support/raw_ostream.h"
 #include <algorithm>
@@ -62,6 +63,17 @@
 using namespace clang::serialization::reader;
 using llvm::BitstreamCursor;
 
+namespace {
+  class PrettyStackTraceASTReader : llvm::PrettyStackTraceString {
+    constexpr static const char * const Message =
+      "\n\n*** Crashed while deserializing from an AST file (e.g. PCH or PCM)."
+      "\n*** Consider clearing your module cache.\n";
+  public:
+    PrettyStackTraceASTReader() : PrettyStackTraceString(Message) {}
+  };
+
+} // end anonymous namespace
+
 
 //===----------------------------------------------------------------------===//
 // ChainedASTReaderListener implementation
@@ -1697,6 +1709,8 @@
 }
 
 void ASTReader::updateOutOfDateIdentifier(IdentifierInfo &II) {
+  PrettyStackTraceASTReader StackTraceRAII;
+
   // Note that we are loading an identifier.
   Deserializing AnIdentifier(this);
 
@@ -3585,6 +3599,7 @@
                                             ModuleKind Type,
                                             SourceLocation ImportLoc,
                                             unsigned ClientLoadCapabilities) {
+  PrettyStackTraceASTReader StackTraceRAII;
   llvm::SaveAndRestore<SourceLocation>
     SetCurImportLocRAII(CurrentImportLoc, ImportLoc);
 
@@ -6054,6 +6069,7 @@
 }
 
 Decl *ASTReader::GetExternalDecl(uint32_t ID) {
+  PrettyStackTraceASTReader StackTraceRAII;
   return GetDecl(ID);
 }
 
@@ -6067,6 +6083,8 @@
     return;
   }
 
+  PrettyStackTraceASTReader StackTraceRAII;
+
   const DeclContext *DC = D->getDeclContext()->getRedeclContext();
 
   // If this is a named declaration, complete it by looking it up
@@ -6105,6 +6123,8 @@
 }
 
 CXXBaseSpecifier *ASTReader::GetExternalCXXBaseSpecifiers(uint64_t Offset) {
+  PrettyStackTraceASTReader StackTraceRAII;
+
   RecordLocation Loc = getLocalBitOffset(Offset);
   BitstreamCursor &Cursor = Loc.F->DeclsCursor;
   SavedStreamPosition SavedPosition(Cursor);
@@ -6274,6 +6294,8 @@
 /// source each time it is called, and is meant to be used via a
 /// LazyOffsetPtr (which is used by Decls for the body of functions, etc).
 Stmt *ASTReader::GetExternalDeclStmt(uint64_t Offset) {
+  PrettyStackTraceASTReader StackTraceRAII;
+
   // Switch case IDs are per Decl.
   ClearSwitchCaseIDs();
 
@@ -6344,6 +6366,8 @@
 ExternalLoadResult ASTReader::FindExternalLexicalDecls(const DeclContext *DC,
                                          bool (*isKindWeWant)(Decl::Kind),
                                          SmallVectorImpl<Decl*> &Decls) {
+  PrettyStackTraceASTReader StackTraceRAII;
+
   // There might be lexical decls in multiple modules, for the TU at
   // least. Walk all of the modules in the order they were loaded.
   FindExternalLexicalDeclsVisitor Visitor(*this, DC, isKindWeWant, Decls);
@@ -6388,6 +6412,8 @@
 void ASTReader::FindFileRegionDecls(FileID File,
                                     unsigned Offset, unsigned Length,
                                     SmallVectorImpl<Decl *> &Decls) {
+  PrettyStackTraceASTReader StackTraceRAII;
+
   SourceManager &SM = getSourceManager();
 
   llvm::DenseMap<FileID, FileDeclsInfo>::iterator I = FileDeclIDs.find(File);
@@ -6528,6 +6554,7 @@
   if (!Name)
     return false;
 
+  PrettyStackTraceASTReader StackTraceRAII;
   Deserializing LookupResults(this);
 
   SmallVector<NamedDecl *, 64> Decls;
@@ -6648,6 +6675,9 @@
 void ASTReader::completeVisibleDeclsMap(const DeclContext *DC) {
   if (!DC->hasExternalVisibleStorage())
     return;
+
+  PrettyStackTraceASTReader StackTraceRAII;
+
   DeclsMap Decls;
 
   // Compute the declaration contexts we need to look into. Multiple such
@@ -7488,6 +7518,7 @@
 }
 
 Module *ASTReader::getModule(unsigned ID) {
+  PrettyStackTraceASTReader StackTraceRAII;
   return getSubmodule(ID);
 }
 
@@ -7521,10 +7552,12 @@
 }
 
 Selector ASTReader::GetExternalSelector(serialization::SelectorID ID) {
+  PrettyStackTraceASTReader StackTraceRAII;
   return DecodeSelector(ID);
 }
 
 uint32_t ASTReader::GetNumExternalSelectors() {
+  PrettyStackTraceASTReader StackTraceRAII;
   // ID 0 (the null selector) is considered an external selector.
   return getTotalNumSelectors() + 1;
 }
@@ -8092,6 +8125,8 @@
 }
 
 void ASTReader::ReadComments() {
+  PrettyStackTraceASTReader StackTraceRAII;
+
   std::vector<RawComment *> Comments;
   for (SmallVectorImpl<std::pair<BitstreamCursor,
                                  serialization::ModuleFile *> >::iterator