Index: clang/include/clang/Basic/DiagnosticSerializationKinds.td
===================================================================
--- clang/include/clang/Basic/DiagnosticSerializationKinds.td
+++ clang/include/clang/Basic/DiagnosticSerializationKinds.td
@@ -74,6 +74,8 @@
   "imported by %select{|module '%2' in }1'%0'">;
 def err_module_file_not_module : Error<
   "AST file '%0' was not built as a module">, DefaultFatal;
+def err_module_file_missing_definition : Error<
+  "module file '%0' is missing the main module's definition">, DefaultFatal;
 
 def remark_module_import : Remark<
   "importing module '%0'%select{| into '%3'}2 from '%1'">,
Index: clang/include/clang/Serialization/Module.h
===================================================================
--- clang/include/clang/Serialization/Module.h
+++ clang/include/clang/Serialization/Module.h
@@ -159,6 +159,9 @@
   /// Whether the PCH has a corresponding object file.
   bool PCHHasObjectFile = false;
 
+  /// Whether the main module has been read from the AST file.
+  bool DidReadMainModule = false;
+
   /// The file entry for the module file.
   const FileEntry *File = nullptr;
 
Index: clang/lib/Serialization/ASTReader.cpp
===================================================================
--- clang/lib/Serialization/ASTReader.cpp
+++ clang/lib/Serialization/ASTReader.cpp
@@ -4219,6 +4219,12 @@
     if (ASTReadResult Result = ReadASTBlock(F, ClientLoadCapabilities))
       return removeModulesAndReturn(Result);
 
+    // The AST block should always have a definition for the main module.
+    if (F.isModule() && !F.DidReadMainModule) {
+      Error(diag::err_module_file_missing_definition, F.FileName);
+      return removeModulesAndReturn(Failure);
+    }
+
     // Read the extension blocks.
     while (!SkipCursorToBlock(F.Stream, EXTENSION_BLOCK_ID)) {
       if (ASTReadResult Result = ReadExtensionBlock(F))
@@ -5494,6 +5500,7 @@
           }
         }
 
+        F.DidReadMainModule = true;
         CurrentModule->setASTFile(F.File);
         CurrentModule->PresumedModuleMapFile = F.ModuleMapPath;
       }