libcpp: Named universal character escapes and delimited escape sequence tweaks

On Tue, Aug 30, 2022 at 09:10:37PM +0000, Joseph Myers wrote:
> I'm seeing build failures of glibc for powerpc64, as illustrated by the
> following C code:
>
> #if 0
> \NARG
> #endif
>
> (the actual sysdeps/powerpc/powerpc64/sysdep.h code is inside #ifdef
> __ASSEMBLER__).
>
> This shows some problems with this feature - and with delimited escape
> sequences - as it affects C.  It's fine to accept it as an extension
> inside string and character literals, because \N or \u{...} would be
> invalid in the absence of the feature (i.e. the syntax for such literals
> fails to match, meaning that the rule about undefined behavior for a
> single ' or " as a pp-token applies).  But outside string and character
> literals, the usual lexing rules apply, the \ is a pp-token on its own and
> the code is valid at the preprocessing level, and with expansion of macros
> appearing before or after the \ (e.g. u defined as a macro in the \u{...}
> case) it may be valid code at the language level as well.  I don't know
> what older C++ versions say about this, but for C this means e.g.
>
> #define z(x) 0
> #define a z(
> int x = a\NARG);
>
> needs to be accepted as expanding to "int x = 0;", not interpreted as
> using the \N feature in an identifier and produce an error.

The following patch changes this, so that:
1) outside of string/character literals, \N without following { is never
   treated as an error nor warning, it is silently treated as \ separate
   token followed by whatever is after it
2) \u{123} and \N{LATIN SMALL LETTER A WITH ACUTE} are not handled as
   extension at all outside of string/character literals in the strict
   standard modes (-std=c*) except for -std=c++{23,2b}, only in the
   -std=gnu* modes, because it changes behavior on valid sources, e.g.
   #define z(x) 0
   #define a z(
   int x = a\u{123});
   int y = a\N{LATIN SMALL LETTER A WITH ACUTE});
3) introduces -Wunicode warning (on by default) and warns for cases
   of what looks like invalid delimited escape sequence or named
   universal character escape outside of string/character literals
   and is treated as separate tokens

2022-09-07  Jakub Jelinek  <jakub@redhat.com>

libcpp/
	* include/cpplib.h (struct cpp_options): Add cpp_warn_unicode member.
	(enum cpp_warning_reason): Add CPP_W_UNICODE.
	* init.cc (cpp_create_reader): Initialize cpp_warn_unicode.
	* charset.cc (_cpp_valid_ucn): In possible identifier contexts, don't
	handle \u{ or \N{ specially in -std=c* modes except -std=c++2{3,b}.
	In possible identifier contexts, don't emit an error and punt
	if \N isn't followed by {, or if \N{} surrounds some lower case
	letters or _.  In possible identifier contexts when not C++23, don't
	emit an error but warning about unknown character names and treat as
	separate tokens.  When treating as separate tokens \u{ or \N{, emit
	warnings.
gcc/
	* doc/invoke.texi (-Wno-unicode): Document.
gcc/c-family/
	* c.opt (Winvalid-utf8): Use ObjC instead of objC.  Remove
	" in comments" from description.
	(Wunicode): New option.
gcc/testsuite/
	* c-c++-common/cpp/delimited-escape-seq-4.c: New test.
	* c-c++-common/cpp/delimited-escape-seq-5.c: New test.
	* c-c++-common/cpp/delimited-escape-seq-6.c: New test.
	* c-c++-common/cpp/delimited-escape-seq-7.c: New test.
	* c-c++-common/cpp/named-universal-char-escape-5.c: New test.
	* c-c++-common/cpp/named-universal-char-escape-6.c: New test.
	* c-c++-common/cpp/named-universal-char-escape-7.c: New test.
	* g++.dg/cpp23/named-universal-char-escape1.C: New test.
	* g++.dg/cpp23/named-universal-char-escape2.C: New test.

libcpp/include/cpplib.h

@@ -565,6 +565,10 @@ struct cpp_options
      2 if it should be a pedwarn.  */
   unsigned char cpp_warn_invalid_utf8;
 
+  /* True if libcpp should warn about invalid forms of delimited or named
+     escape sequences.  */
+  bool cpp_warn_unicode;
+
   /* True if -finput-charset= option has been used explicitly.  */
   bool cpp_input_charset_explicit;
 
@@ -675,7 +679,8 @@ enum cpp_warning_reason {
   CPP_W_CXX20_COMPAT,
   CPP_W_EXPANSION_TO_DEFINED,
   CPP_W_BIDIRECTIONAL,
-  CPP_W_INVALID_UTF8
+  CPP_W_INVALID_UTF8,
+  CPP_W_UNICODE
 };
 
 /* Callback for header lookup for HEADER, which is the name of a