PR c/83656 - missing -Wbuiltin-declaration-mismatch on declaration without prototype

gcc/c/ChangeLog:

	PR c/83656
	* c-decl.c (diagnose_mismatched_decls): Diagnose declarations of
	built-in functions without a prototype.

gcc/testsuite/ChangeLog:

	PR c/83656
	* gcc.dg/Wbuiltin-declaration-mismatch-5.c: New test.

diff --git a/gcc/c/c-decl.c b/gcc/c/c-decl.c
index 6c9e667..7008176 100644
--- a/gcc/c/c-decl.c
+++ b/gcc/c/c-decl.c
@@ -2088,15 +2088,29 @@ diagnose_mismatched_decls (tree newdecl, tree olddecl,
 	 can't validate the argument list) the built-in definition is
 	 overridden, but optionally warn this was a bad choice of name.  */
       if (DECL_BUILT_IN (olddecl)
-	  && !C_DECL_DECLARED_BUILTIN (olddecl)
-	  && (!TREE_PUBLIC (newdecl)
-	      || (DECL_INITIAL (newdecl)
-		  && !prototype_p (TREE_TYPE (newdecl)))))
+	  && !C_DECL_DECLARED_BUILTIN (olddecl))
 	{
-	  warning (OPT_Wshadow, "declaration of %q+D shadows "
-		   "a built-in function", newdecl);
-	  /* Discard the old built-in function.  */
-	  return false;
+	  if (!TREE_PUBLIC (newdecl)
+	      || (DECL_INITIAL (newdecl)
+		  && !prototype_p (TREE_TYPE (newdecl))))
+	    {
+	      warning_at (DECL_SOURCE_LOCATION (newdecl),
+			  OPT_Wshadow, "declaration of %qD shadows "
+			  "a built-in function", newdecl);
+	      /* Discard the old built-in function.  */
+	      return false;
+	    }
+
+	  if (!prototype_p (TREE_TYPE (newdecl)))
+	    {
+	      warning_at (DECL_SOURCE_LOCATION (newdecl),
+			  OPT_Wbuiltin_declaration_mismatch,
+			  "declaration of built-in function %qD without "
+			  "a prototype; expected %qT",
+			  newdecl, TREE_TYPE (olddecl));
+	      /* Discard the old built-in function.  */
+	      return false;
+	    }
 	}
 
       if (DECL_INITIAL (newdecl))
diff --git a/gcc/testsuite/gcc.dg/Wbuiltin-declaration-mismatch-5.c b/gcc/testsuite/gcc.dg/Wbuiltin-declaration-mismatch-5.c
new file mode 100644
index 0000000..51a634d
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/Wbuiltin-declaration-mismatch-5.c
@@ -0,0 +1,10 @@
+/* PR c/83656 - missing -Wbuiltin-declaration-mismatch on declaration
+   without prototype
+   { dg-do compile }
+   { dg-options "-Wbuiltin-declaration-mismatch" } */
+
+void* memcpy ();   /* { dg-warning "declaration of built-in function .memcpy. without a prototype; expected .void \\\*\\\(void \\\*, const void \\\*, \(long \)?unsigned int\\\)." } */
+
+int strcmp ();   /* { dg-warning "declaration of built-in function .strcmp. without a prototype; expected .int\\\(const char* \\\*, const char \\\*\\\)." } */
+
+int strcpy ();   /* { dg-warning "conflicting types for built-in function .strcpy.; expected .char \\\*\\\(char \\\*, const char \\\*\\\)." } */