Make diagnostics.pm understand messages sharing descriptions

We currently have entries in perldiag like this:

=item Code point 0x%X is not Unicode, may not be portable

=item Code point 0x%X is not Unicode, no properties match it; all inverse properties do

(W utf8) You had a code point above the Unicode maximum of U+10FFFF.

...

diagnostics.pm needs to know that the description applies to the first
=item, as well as to the second.

1 files changed, 15 insertions, 0 deletions

diff --git a/lib/diagnostics.pm b/lib/diagnostics.pm
index cd4e7b6151..b3464488d7 100644
--- a/lib/diagnostics.pm
+++ b/lib/diagnostics.pm
@@ -319,7 +319,9 @@ my %msg;
     print STDERR "FINISHING COMPILATION for $_\n" if $DEBUG;
     local $/ = '';
     my $header;
+    my @headers;
     my $for_item;
+    my $seen_body;
     while (<POD_DIAG>) {
 
 	sub _split_pod_link {
@@ -365,10 +367,22 @@ my %msg;
 		}
 		s/^/    /gm;
 		$msg{$header} .= $_;
+		for my $h(@headers) { $msg{$h} .= $_ }
+		++$seen_body;
 	 	undef $for_item;	
 	    }
 	    next;
 	} 
+
+	# If we have not come across the body of the description yet, then
+	# the previous header needs to share the same description.
+	if ($seen_body) {
+	    @headers = ();
+	}
+	else {
+	    push @headers, $header if defined $header;
+	}
+
 	unless ( s/=item (.*?)\s*\z//) {
 
 	    if ( s/=head1\sDESCRIPTION//) {
@@ -428,6 +442,7 @@ my %msg;
 	    if $msg{$header};
 
 	$msg{$header} = '';
+	$seen_body = 0;
     }