diff --git a/clang/include/clang/Serialization/ASTBitCodes.h b/clang/include/clang/Serialization/ASTBitCodes.h
--- a/clang/include/clang/Serialization/ASTBitCodes.h
+++ b/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.
 ///
diff --git a/clang/lib/Serialization/ASTReader.cpp b/clang/lib/Serialization/ASTReader.cpp
--- a/clang/lib/Serialization/ASTReader.cpp
+++ b/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:
diff --git a/clang/test/Modules/embed-files-compressed.cpp b/clang/test/Modules/embed-files-compressed.cpp
--- a/clang/test/Modules/embed-files-compressed.cpp
+++ b/clang/test/Modules/embed-files-compressed.cpp
@@ -17,7 +17,7 @@
 // RUN: %clang_cc1 -fmodules -I%t -fmodules-cache-path=%t -fmodule-name=a -emit-module %t/modulemap -fmodules-embed-all-files -o %t/a.pcm
 //
 // The above embeds ~4.5MB of highly-predictable /s and \ns into the pcm file.
-// Check that the resulting file is under 40KB:
+// Check that the resulting file is under 60KB:
 //
 // RUN: wc -c %t/a.pcm | FileCheck --check-prefix=CHECK-SIZE %s
-// CHECK-SIZE: {{(^|[^0-9])[123][0-9][0-9][0-9][0-9]($|[^0-9])}}
+// CHECK-SIZE: {{(^|[^0-9])[1-5][0-9][0-9][0-9][0-9]($|[^0-9])}}
diff --git a/clang/test/Modules/empty.modulemap b/clang/test/Modules/empty.modulemap
--- a/clang/test/Modules/empty.modulemap
+++ b/clang/test/Modules/empty.modulemap
@@ -13,8 +13,8 @@
 // The module file should be identical each time we produce it.
 // RUN: diff %t/base.pcm %t/check.pcm
 //
-// We expect an empty module to be less than 40KB (and at least 10K, for now).
+// We expect an empty module to be less than 60KB (and at least 10K, for now).
 // RUN: wc -c %t/base.pcm | FileCheck --check-prefix=CHECK-SIZE %s
-// CHECK-SIZE: {{(^|[^0-9])[123][0-9][0-9][0-9][0-9]($|[^0-9])}}
+// CHECK-SIZE: {{(^|[^0-9])[1-5][0-9][0-9][0-9][0-9]($|[^0-9])}}
 
 module empty { header "Inputs/empty.h" export * }