2 files changed, 16 insertions, 0 deletions
diff --git a/clang/lib/Serialization/ASTReader.cpp b/clang/lib/Serialization/ASTReader.cpp
index 5c25f584038..c0976ee1e65 100644
--- a/clang/lib/Serialization/ASTReader.cpp
+++ b/clang/lib/Serialization/ASTReader.cpp
@@ -2866,6 +2866,9 @@ ASTReader::ReadASTCore(StringRef FileName,
     return Failure;
   }
 
+  // This is used for compatibility with older PCH formats.
+  bool HaveReadControlBlock = false;
+
   while (!Stream.AtEndOfStream()) {
     unsigned Code = Stream.ReadCode();
 
@@ -2885,6 +2888,7 @@ ASTReader::ReadASTCore(StringRef FileName,
       }
       break;
     case CONTROL_BLOCK_ID:
+      HaveReadControlBlock = true;
       switch (ReadControlBlock(F, Loaded, ClientLoadCapabilities)) {
       case Success:
         break;
@@ -2897,6 +2901,12 @@ ASTReader::ReadASTCore(StringRef FileName,
       }
       break;
     case AST_BLOCK_ID:
+      if (!HaveReadControlBlock) {
+        if ((ClientLoadCapabilities & ARR_VersionMismatch) == 0)
+          Diag(diag::warn_pch_version_too_old);
+        return VersionMismatch;
+      }
+
       // Record that we've loaded this module.
       Loaded.push_back(ImportedModule(M, ImportedBy, ImportLoc));
       return Success;
diff --git a/clang/lib/Serialization/ASTWriter.cpp b/clang/lib/Serialization/ASTWriter.cpp
index ed0a272faa0..1b39f551bf2 100644
--- a/clang/lib/Serialization/ASTWriter.cpp
+++ b/clang/lib/Serialization/ASTWriter.cpp
@@ -3568,6 +3568,12 @@ void ASTWriter::WriteASTCore(Sema &SemaRef,
   RecordData Record;
   Stream.EnterSubblock(AST_BLOCK_ID, 5);
 
+  // This is so that older clang versions, before the introduction
+  // of the control block, can read and reject the newer PCH format.
+  Record.clear();
+  Record.push_back(VERSION_MAJOR);
+  Stream.EmitRecord(METADATA_OLD_FORMAT, Record);
+
   // Create a lexical update block containing all of the declarations in the
   // translation unit that do not come from other AST files.
   const TranslationUnitDecl *TU = Context.getTranslationUnitDecl();