libcpp: Add -Winvalid-utf8 warning [PR106655]

The following patch introduces a new warning - -Winvalid-utf8 similarly
to what clang now has - to diagnose invalid UTF-8 byte sequences in
comments, but not just in those, but also in string/character literals
and outside of them.

The warning is on by default when explicit -finput-charset=UTF-8 is
used and C++23 compilation is requested and if -{,W}pedantic or
-pedantic-errors it is actually a pedwarn.

The reason it is on by default only for -finput-charset=UTF-8 is
that the sources often are UTF-8, but sometimes could be some ASCII
compatible single byte encoding where non-ASCII characters only
appear in comments.  So having the warning off by default
is IMO desirable.  The C++23 pedantic mode for when the source code
is UTF-8 is -std=c++23 -pedantic-errors -finput-charset=UTF-8.

2022-09-01  Jakub Jelinek  <jakub@redhat.com>

	PR c++/106655
libcpp/
	* include/cpplib.h (struct cpp_options): Implement C++23
	P2295R6 - Support for UTF-8 as a portable source file encoding.
	Add cpp_warn_invalid_utf8 and cpp_input_charset_explicit fields.
	(enum cpp_warning_reason): Add CPP_W_INVALID_UTF8 enumerator.
	* init.cc (cpp_create_reader): Initialize cpp_warn_invalid_utf8
	and cpp_input_charset_explicit.
	* charset.cc (_cpp_valid_utf8): Adjust function comment.
	* lex.cc (UCS_LIMIT): Define.
	(utf8_continuation): New const variable.
	(utf8_signifier): Move earlier in the file.
	(_cpp_warn_invalid_utf8, _cpp_handle_multibyte_utf8): New functions.
	(_cpp_skip_block_comment): Handle -Winvalid-utf8 warning.
	(skip_line_comment): Likewise.
	(lex_raw_string, lex_string): Likewise.
	(_cpp_lex_direct): Likewise.
gcc/
	* doc/invoke.texi (-Winvalid-utf8): Document it.
gcc/c-family/
	* c.opt (-Winvalid-utf8): New warning.
	* c-opts.cc (c_common_handle_option) <case OPT_finput_charset_>:
	Set cpp_opts->cpp_input_charset_explicit.
	(c_common_post_options): If -finput-charset=UTF-8 is explicit
	in C++23, enable -Winvalid-utf8 by default and if -pedantic
	or -pedantic-errors, make it a pedwarn.
gcc/testsuite/
	* c-c++-common/cpp/Winvalid-utf8-1.c: New test.
	* c-c++-common/cpp/Winvalid-utf8-2.c: New test.
	* c-c++-common/cpp/Winvalid-utf8-3.c: New test.
	* g++.dg/cpp23/Winvalid-utf8-1.C: New test.
	* g++.dg/cpp23/Winvalid-utf8-2.C: New test.
	* g++.dg/cpp23/Winvalid-utf8-3.C: New test.
	* g++.dg/cpp23/Winvalid-utf8-4.C: New test.
	* g++.dg/cpp23/Winvalid-utf8-5.C: New test.
	* g++.dg/cpp23/Winvalid-utf8-6.C: New test.
	* g++.dg/cpp23/Winvalid-utf8-7.C: New test.
	* g++.dg/cpp23/Winvalid-utf8-8.C: New test.
	* g++.dg/cpp23/Winvalid-utf8-9.C: New test.
	* g++.dg/cpp23/Winvalid-utf8-10.C: New test.
	* g++.dg/cpp23/Winvalid-utf8-11.C: New test.
	* g++.dg/cpp23/Winvalid-utf8-12.C: New test.

1 files changed, 12 insertions, 0 deletions

diff --git a/gcc/c-family/c-opts.cc b/gcc/c-family/c-opts.cc
index 337a524..babaa2f 100644
--- a/gcc/c-family/c-opts.cc
+++ b/gcc/c-family/c-opts.cc
@@ -534,6 +534,7 @@ c_common_handle_option (size_t scode, const char *arg, HOST_WIDE_INT value,
 
     case OPT_finput_charset_:
       cpp_opts->input_charset = arg;
+      cpp_opts->cpp_input_charset_explicit = 1;
       break;
 
     case OPT_ftemplate_depth_:
@@ -1152,6 +1153,17 @@ c_common_post_options (const char **pfilename)
     lang_hooks.preprocess_options (parse_in);
   cpp_post_options (parse_in);
   init_global_opts_from_cpp (&global_options, cpp_get_options (parse_in));
+  /* For C++23 and explicit -finput-charset=UTF-8, turn on -Winvalid-utf8
+     by default and make it a pedwarn unless -Wno-invalid-utf8.  */
+  if (cxx_dialect >= cxx23
+      && cpp_opts->cpp_input_charset_explicit
+      && strcmp (cpp_opts->input_charset, "UTF-8") == 0
+      && (cpp_opts->cpp_warn_invalid_utf8
+	  || !global_options_set.x_warn_invalid_utf8))
+    {
+      global_options.x_warn_invalid_utf8 = 1;
+      cpp_opts->cpp_warn_invalid_utf8 = cpp_opts->cpp_pedantic ? 2 : 1;
+    }
 
   /* Let diagnostics infrastructure know how to convert input files the same
      way libcpp will do it, namely using the configured input charset and