Index: clang/include/clang/Serialization/ASTBitCodes.h
===================================================================
--- clang/include/clang/Serialization/ASTBitCodes.h
+++ clang/include/clang/Serialization/ASTBitCodes.h
@@ -1099,6 +1099,8 @@
 // \brief WebAssembly reference types with auto numeration
 #define WASM_TYPE(Name, Id, SingletonId) PREDEF_TYPE_##Id##_ID,
 #include "clang/Basic/WebAssemblyReferenceTypes.def"
+  // Sentinel value. Considered a predefined type but not useable as one.
+  PREDEF_TYPE_LAST_ID
 };
 
 /// The number of predefined type IDs that are reserved for
@@ -1106,7 +1108,13 @@
 ///
 /// Type IDs for non-predefined types will start at
 /// NUM_PREDEF_TYPE_IDs.
-const unsigned NUM_PREDEF_TYPE_IDS = 300;
+const unsigned NUM_PREDEF_TYPE_IDS = 500;
+
+// Ensure we do not overrun the predefined types we reserved
+// in the enum PredefinedTypeIDs above.
+static_assert(PREDEF_TYPE_LAST_ID < NUM_PREDEF_TYPE_IDS,
+              "Too many enumerators in PredefinedTypeIDs. Review the value of "
+              "NUM_PREDEF_TYPE_IDS");
 
 /// Record codes for each kind of type.
 ///
Index: clang/lib/Serialization/ASTReader.cpp
===================================================================
--- clang/lib/Serialization/ASTReader.cpp
+++ clang/lib/Serialization/ASTReader.cpp
@@ -6983,6 +6983,10 @@
   if (Index < NUM_PREDEF_TYPE_IDS) {
     QualType T;
     switch ((PredefinedTypeIDs)Index) {
+    case PREDEF_TYPE_LAST_ID:
+      // We should never use this one.
+      llvm_unreachable("Invalid predefined type");
+      break;
     case PREDEF_TYPE_NULL_ID:
       return QualType();
     case PREDEF_TYPE_VOID_ID: