Emit error for missing format attribute

Author: mysterymath

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -13017,6 +13017,9 @@ def err_get_vtable_pointer_requires_complete_type
     : Error<"__builtin_get_vtable_pointer requires an argument with a complete "
             "type, but %0 is incomplete">;
 
+def err_modular_format_attribute_no_format : Error<
+    "'modular_format' attribute requires 'format' attribute">;
+
 // SYCL-specific diagnostics
 def warn_sycl_kernel_num_of_template_params : Warning<
   "'sycl_kernel' attribute only applies to a function template with at least"

clang/lib/Sema/SemaDecl.cpp

@@ -7210,6 +7210,11 @@ static void checkLifetimeBoundAttr(Sema &S, NamedDecl &ND) {
   }
 }
 
+static void checkModularFormatAttr(Sema &S, NamedDecl &ND) {
+  if (ND.hasAttr<ModularFormatAttr>() && !ND.hasAttr<FormatAttr>())
+    S.Diag(ND.getLocation(), diag::err_modular_format_attribute_no_format);
+}
+
 static void checkAttributesAfterMerging(Sema &S, NamedDecl &ND) {
   // Ensure that an auto decl is deduced otherwise the checks below might cache
   // the wrong linkage.
@@ -7222,6 +7227,7 @@ static void checkAttributesAfterMerging(Sema &S, NamedDecl &ND) {
   checkHybridPatchableAttr(S, ND);
   checkInheritableAttr(S, ND);
   checkLifetimeBoundAttr(S, ND);
+  checkModularFormatAttr(S, ND);
 }
 
 static void checkDLLAttributeRedeclaration(Sema &S, NamedDecl *OldDecl,

clang/test/Sema/attr-modular-format.c

@@ -0,0 +1,7 @@
+//RUN: %clang_cc1 -fsyntax-only -verify %s
+
+#include <stdarg.h>
+
+int printf(const char *fmt, ...)  __attribute__((modular_format(__modular_printf, "__printf", "float")));  // no-error
+int myprintf(const char *fmt, ...)  __attribute__((modular_format(__modular_printf, "__printf", "float")));  // expected-error {{'modular_format' attribute requires 'format' attribute}}
+