Index: include/clang/Basic/DiagnosticSemaKinds.td
===================================================================
--- include/clang/Basic/DiagnosticSemaKinds.td
+++ include/clang/Basic/DiagnosticSemaKinds.td
@@ -7919,6 +7919,8 @@
   "'type_tag_for_datatype' attribute requires the initializer to be "
   "an %select{integer|integral}0 constant expression "
   "that can be represented by a 64 bit integer">;
+def err_tag_index_out_of_range : Error<
+  "%select{type tag|argument}0 index %1 is greater than the number of arguments specified">;
 def warn_type_tag_for_datatype_wrong_kind : Warning<
   "this type tag was not designed to be used with this function">,
   InGroup<TypeSafety>;
Index: include/clang/Sema/Sema.h
===================================================================
--- include/clang/Sema/Sema.h
+++ include/clang/Sema/Sema.h
@@ -10455,7 +10455,8 @@
   /// \brief Peform checks on a call of a function with argument_with_type_tag
   /// or pointer_with_type_tag attributes.
   void CheckArgumentWithTypeTag(const ArgumentWithTypeTagAttr *Attr,
-                                const Expr * const *ExprArgs);
+                                const ArrayRef<const Expr *> ExprArgs,
+                                SourceLocation CallSiteLoc);
 
   /// \brief Check if we are taking the address of a packed field
   /// as this may be a problem if the pointer value is dereferenced.
Index: lib/Sema/SemaChecking.cpp
===================================================================
--- lib/Sema/SemaChecking.cpp
+++ lib/Sema/SemaChecking.cpp
@@ -2754,7 +2754,7 @@
     // Type safety checking.
     if (FDecl) {
       for (const auto *I : FDecl->specific_attrs<ArgumentWithTypeTagAttr>())
-        CheckArgumentWithTypeTag(I, Args.data());
+        CheckArgumentWithTypeTag(I, Args, Loc);
     }
   }
 
@@ -12329,10 +12329,18 @@
 }
 
 void Sema::CheckArgumentWithTypeTag(const ArgumentWithTypeTagAttr *Attr,
-                                    const Expr * const *ExprArgs) {
+                                    const ArrayRef<const Expr *> ExprArgs,
+                                    SourceLocation CallSiteLoc) {
   const IdentifierInfo *ArgumentKind = Attr->getArgumentKind();
   bool IsPointerAttr = Attr->getIsPointer();
 
+  // Retrieve the argument representing the 'type_tag'.
+  if (Attr->getTypeTagIdx() >= ExprArgs.size()) {
+    // Add 1 to display the user's specified value.
+    Diag(CallSiteLoc, diag::err_tag_index_out_of_range)
+        << 0 << Attr->getTypeTagIdx() + 1;
+    return;
+  }
   const Expr *TypeTagExpr = ExprArgs[Attr->getTypeTagIdx()];
   bool FoundWrongKind;
   TypeTagData TypeInfo;
@@ -12346,6 +12354,13 @@
     return;
   }
 
+  // Retrieve the argument representing the 'arg_idx'.
+  if (Attr->getArgumentIdx() >= ExprArgs.size()) {
+    // Add 1 to display the user's specified value.
+    Diag(CallSiteLoc, diag::err_tag_index_out_of_range)
+        << 1 << Attr->getArgumentIdx() + 1;
+    return;
+  }
   const Expr *ArgumentExpr = ExprArgs[Attr->getArgumentIdx()];
   if (IsPointerAttr) {
     // Skip implicit cast of pointer to `void *' (as a function argument).
Index: test/Sema/error-type-safety.cpp
===================================================================
--- test/Sema/error-type-safety.cpp
+++ test/Sema/error-type-safety.cpp
@@ -0,0 +1,23 @@
+// RUN: %clang_cc1 -fsyntax-only -verify %s
+
+#define INT_TAG 42
+
+static const int test_in
+  __attribute__((type_tag_for_datatype(test, int))) = INT_TAG;
+
+// Argument index: 1, Type tag index: 2
+void test_bounds_index(...)
+  __attribute__((argument_with_type_tag(test, 1, 2)));
+
+// Argument index: 3, Type tag index: 1
+void test_bounds_arg_index(...)
+  __attribute__((argument_with_type_tag(test, 3, 1)));
+
+void test_bounds()
+{
+  // Test the boundary edges (ensure no off-by-one) with argument indexing.
+  test_bounds_index(1, INT_TAG);
+
+  test_bounds_index(1); // expected-error {{type tag index 2 is greater than the number of arguments specified}}
+  test_bounds_arg_index(INT_TAG, 1); // expected-error {{argument index 3 is greater than the number of arguments specified}}
+}