Reinstate "Raise warnings for poorly supported locales"

This reverts commit 1244bd171b8d1fd4b6179e537f7b95c38bd8f099,
thus reinstating commit 3d3a881c1b0eb9c855d257a2eea1f72666e30fbc.

1 files changed, 19 insertions, 2 deletions

diff --git a/pod/perllocale.pod b/pod/perllocale.pod
index c693ecb2aa..d083c09d2f 100644
--- a/pod/perllocale.pod
+++ b/pod/perllocale.pod
@@ -863,12 +863,18 @@ example, if you move from the "C" locale to a 7-bit ISO 646 one,
 you may find--possibly to your surprise--that C<"|"> moves from the
 C<POSIX::ispunct()> class to C<POSIX::isalpha()>.
 Unfortunately, this creates big problems for regular expressions. "|" still
-means alternation even though it matches C<\w>.
+means alternation even though it matches C<\w>.  Starting in v5.22, a
+warning will be raised when such a locale is switched into.  More
+details are given several paragraphs further down.
 
 Starting in v5.20, Perl supports UTF-8 locales for C<LC_CTYPE>, but
 otherwise Perl only supports single-byte locales, such as the ISO 8859
 series.  This means that wide character locales, for example for Asian
-languages, are not supported.  The UTF-8 locale support is actually a
+languages, are not well-supported.  (If the platform has the capability
+for Perl to detect such a locale, starting in Perl v5.22,
+L<Perl will warn, default enabled|warnings/Category Hierarchy>,
+using the C<locale> warning category, whenever such a locale is switched
+into.)  The UTF-8 locale support is actually a
 superset of POSIX locales, because it is really full Unicode behavior
 as if no locale were in effect at all (except for tainting; see
 L</SECURITY>).  POSIX locales, even UTF-8 ones,
@@ -890,6 +896,17 @@ C<\n> for example, always mean the platform's native one.  This means,
 for example, that C<\N> in regular expressions (every character
 but new-line) works on the platform character set.
 
+Starting in v5.22, Perl will by default warn when switching into a
+locale that redefines any ASCII printable character (plus C<\t> and
+C<\n>) into a different class than expected.  This is unlikely to
+happen on modern locales, but can happen with the ISO 646 and other
+7-bit locales that are essentially obsolete.  Things may still work,
+depending on what features of Perl are used by the program.  For
+example, in the example from above where C<"|"> becomes a C<\w>, and
+there are no regular expressions where this matters, the program may
+still work properly.  The warning lists all the characters that
+it can determine could be adversely affected.
+
 B<Note:> A broken or malicious C<LC_CTYPE> locale definition may result
 in clearly ineligible characters being considered to be alphanumeric by
 your application.  For strict matching of (mundane) ASCII letters and