[perl #128769] Improve base.pm @INC '.' handling

- Localise @INC only if necessary.
- Don't mention '.' in the @INC list in the error message, since it
  was not in the @INC that was searched (this is accomplished by local-
  ising @INC in the same scope as the error).
- If a file exists that would have been loaded had '.' not been
  ignored, mention it and suggest 'use lib'.
- Use the same number of closing as opening parentheses in the
  error message.

(cherry picked from commit bca552795994a553e07b38a6f82a233533919926)

3 files changed, 31 insertions, 3 deletions

diff --git a/MANIFEST b/MANIFEST
index a8d68546f8..48b739424e 100644
--- a/MANIFEST
+++ b/MANIFEST
@@ -2892,6 +2892,7 @@ dist/base/t/fields-5_6_0.t	See if fields work
 dist/base/t/fields-5_8_0.t	See if fields work
 dist/base/t/fields-base.t	See if fields work
 dist/base/t/fields.t		See if fields work
+dist/base/t/incdoc.t		Test how base.pm handles '.' in @INC
 dist/base/t/isa.t		See if base's behaviour doesn't change
 dist/base/t/lib/Broken.pm	Test module for base.pm
 dist/base/t/lib/Dummy.pm	Test module for base.pm
diff --git a/dist/base/lib/base.pm b/dist/base/lib/base.pm
index ff1a16c1f7..8dc62bbd1c 100644
--- a/dist/base/lib/base.pm
+++ b/dist/base/lib/base.pm
@@ -96,9 +96,9 @@ sub import {
             {
                 local $SIG{__DIE__};
                 my $fn = _module_to_filename($base);
+                my $localinc = $INC[-1] eq '.';
+                local @INC = @INC[0..$#INC-1] if $localinc;
                 eval {
-                    local @INC = @INC;
-                    pop @INC if $INC[-1] eq '.';
                     require $fn
                 };
                 # Only ignore "Can't locate" errors from our eval require.
@@ -114,11 +114,19 @@ sub import {
                 unless (%{"$base\::"}) {
                     require Carp;
                     local $" = " ";
-                    Carp::croak(<<ERROR);
+                    my $e = <<ERROR;
 Base class package "$base" is empty.
     (Perhaps you need to 'use' the module which defines that package first,
     or make that module available in \@INC (\@INC contains: @INC).
 ERROR
+                    if ($localinc && -e $fn) {
+                        $e .= <<ERROS;
+    If you mean to load $fn from the current directory, you may
+    want to try "use lib '.'".
+ERROS
+                    }
+                    $e =~ s/\n\z/)\n/;
+                    Carp::croak($e);
                 }
                 $sigdie = $SIG{__DIE__} || undef;
             }
diff --git a/dist/base/t/incdot.t b/dist/base/t/incdot.t
new file mode 100644
index 0000000000..fadebc4a45
--- /dev/null
+++ b/dist/base/t/incdot.t
@@ -0,0 +1,19 @@
+#!/usr/bin/perl -w
+
+use strict;
+
+use base ();
+
+use Test::More tests => 2;
+
+if ($INC[-1] ne '.') { push @INC, '.' }
+
+my $inc = quotemeta "@INC[0..$#INC-1]";
+
+eval { 'base'->import("foo") };
+like $@, qr/\@INC contains: $inc\).\)/,
+    'Error does not list final dot in @INC (or mention use lib)';
+eval { 'base'->import('t::lib::Dummy') };
+like $@, qr<\@INC contains: $inc\).\n(?x:
+           )    If you mean to load t/lib/Dummy\.pm from the current >,
+    'special cur dir message for existing files in . that are ignored';