c-family: Prevent -Wformat warnings with u8 strings [PR105626]

The <https://gcc.gnu.org/pipermail/gcc/2022-May/238679.html> thread
seems to have concluded that -Wformat shouldn't warn about

  printf((const char*) u8"test %d\n", 1);

saying "format string is not an array of type 'char'".  This code
is not an aliasing violation, and there are no I/O functions for u8
strings, so the const char * cast is OK and shouldn't be disregarded.

	PR c++/105626

gcc/c-family/ChangeLog:

	* c-format.cc (check_format_arg): Don't emit -Wformat warnings with
	u8 strings.

gcc/testsuite/ChangeLog:

	* g++.dg/warn/Wformat-char8_t-1.C: New test.

(cherry picked from commit 543828e79bfa63ef26b11a2c9ea81fd7905f33aa)

diff --git a/gcc/c-family/c-format.cc b/gcc/c-family/c-format.cc
index 98f28c0..ebdac57 100644 (file)
--- a/gcc/c-family/c-format.cc
+++ b/gcc/c-family/c-format.cc
@@ -1748,7 +1748,8 @@ check_format_arg (void *ctx, tree format_tree,
     }
   tree underlying_type
     = TYPE_MAIN_VARIANT (TREE_TYPE (TREE_TYPE (format_tree)));
-  if (underlying_type != char_type_node)
+  if (underlying_type != char_type_node
+      && !(flag_char8_t && underlying_type == char8_type_node))
     {
       if (underlying_type == char16_type_node
          || underlying_type == char32_type_node

diff --git a/gcc/testsuite/g++.dg/warn/Wformat-char8_t-1.C b/gcc/testsuite/g++.dg/warn/Wformat-char8_t-1.C
new file mode 100644 (file)
index 0000000..ba6f388
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wformat-char8_t-1.C
@@ -0,0 +1,10 @@
+// PR c++/105626
+// { dg-do compile { target c++11 } }
+// { dg-options "-Wformat" }
+// { dg-additional-options "-fchar8_t" { target c++17_down } }
+
+int main()
+{
+  __builtin_printf((const char*) u8"test %d\n", 1); // { dg-bogus "format string" }
+  return 0;
+}