Emit a warning if HTTP status code implies success but X-Died header has been set

.travis.yml

@@ -1,5 +1,6 @@
 language: perl
 perl:
+  - "5.26"
   - "5.24"
   - "5.22"
   - "5.20"

cpanfile

@@ -23,10 +23,14 @@ requires "strict" => "0";
 requires "warnings" => "0";
 
 on 'test' => sub {
+  requires "Cwd" => "0";
   requires "ExtUtils::MakeMaker" => "0";
   requires "File::Spec" => "0";
+  requires "File::Temp" => "0";
   requires "PerlIO::encoding" => "0";
   requires "Test::More" => "0.88";
+  requires "Test::Needs" => "0";
+  requires "Test::Warnings" => "0";
   requires "Time::Local" => "0";
   requires "Try::Tiny" => "0";
   requires "perl" => "5.008001";

lib/HTTP/Response.pm

@@ -215,7 +215,19 @@ sub dump
 
 
 sub is_info     { HTTP::Status::is_info     (shift->{'_rc'}); }
-sub is_success  { HTTP::Status::is_success  (shift->{'_rc'}); }
+sub is_success {
+    my $self = shift;
+    if ( HTTP::Status::is_success( $self->{'_rc'} )
+        && $self->header( 'X-Died' ) )
+    {
+        my $warning = <<'EOF';
+The HTTP status code implies success, but the X-Died header has been set
+internally. Something has gone wrong: '
+EOF
+        warn $warning . $self->header('X-Died');
+    }
+    return HTTP::Status::is_success( $self->{'_rc'} );
+}
 sub is_redirect { HTTP::Status::is_redirect (shift->{'_rc'}); }
 sub is_error    { HTTP::Status::is_error    (shift->{'_rc'}); }
 sub is_client_error { HTTP::Status::is_client_error (shift->{'_rc'}); }

t/x-died.t

@@ -0,0 +1,20 @@
+#!/usr/bin/perl -w
+
+use strict;
+use warnings;
+
+use Cwd 'realpath';
+use File::Temp 'tempfile';
+use Test::Needs { 'LWP::UserAgent' => 6.05 };
+use Test::More;
+use Test::Warnings ':all';
+
+my($tmpfh,$tmpfile) = tempfile(UNLINK => 1);
+close $tmpfh;
+chmod 0400, $tmpfile or die $!;
+
+my $res = LWP::UserAgent->new->get('file://' . realpath($0), ':content_file' => $tmpfile);
+ok $res->header('X-Died'), 'X-Died header seen';
+like(warning { $res->is_success }, qr{X\-Died}, 'warning about X-Died header seen.');
+
+done_testing();