Re: [PATCH] c++, v2: Implement C++23 P2071R2 - Named universal character escapes [PR106648]

[Jakub Jelinek <jakub@redhat.com>]

On Wed, Aug 31, 2022 at 10:52:49AM -0400, Jason Merrill wrote:
> It could be more explicit, but I think we can assume that from the existing
> wording; it says it designates the named character.  If there is no such
> character, that cannot be satisfied, so it must be ill-formed.

Ok.

> > So, we could reject the int h case above and accept silently the others?
> 
> Why not warn on the others?

We were always silent for the cases like \u123X or \U12345X.
Do you think we should emit some warnings (but never pedwarns/errors in that
case) that it is universal character name like but not completely?

The following patch let's us silently accept:
#define z(x) 0
#define a z(
int b = a\u{});
int c = a\u{);
int d = a\N{});
int e = a\N{);
int f = a\u123);
int g = a\U1234567);
int h = a\N);
int i = a\NARG);
int j = a\N{abc});
int k = a\N{ABC.123});

The following 2 will be still rejected with errors:
int l = a\N{ABC});
int m = a\N{LATIN SMALL LETTER A WITH ACUTE});
the first one because ABC is not valid Unicode name and the latter because
it will be int m = aá); and will trigger other errors later.

Given what you said above, I think that is what we want for the last 2
for C++23, the question is if it is ok also for C++20/C17 etc. and whether
it should depend on -pedantic or -pedantic-errors or GNU vs. ISO mode
or not in that case.  We could handle those 2 also differently, just
warn instead of error for the \N{ABC} case if not in C++23 mode when
identifier_pos.

--- libcpp/charset.cc.jj	2022-08-31 12:34:18.921176118 +0200
+++ libcpp/charset.cc	2022-08-31 16:50:48.862775486 +0200
@@ -1463,7 +1463,14 @@ _cpp_valid_ucn (cpp_reader *pfile, const
     {
       length = 4;
       if (str == limit || *str != '{')
-	cpp_error (pfile, CPP_DL_ERROR, "'\\N' not followed by '{'");
+	{
+	  if (identifier_pos)
+	    {
+	      *cp = 0;
+	      return false;
+	    }
+	  cpp_error (pfile, CPP_DL_ERROR, "'\\N' not followed by '{'");
+	}
       else
 	{
 	  str++;
@@ -1489,7 +1496,7 @@ _cpp_valid_ucn (cpp_reader *pfile, const
 
 	  if (str < limit && *str == '}')
 	    {
-	      if (name == str && identifier_pos)
+	      if (identifier_pos && (name == str || !strict))
 		{
 		  *cp = 0;
 		  return false;

	Jakub