check for existence of sub naming subs before using them

Sub::Util ships with new versions of List::Util, but its subs are
provided by the XS component of List::Util.  All Sub::Name itself does
is load List::Util to ensure the subs are available.

If List::Util is downgraded or the new version is otherwise unavailable,
Sub::Util will load fine but the subs won't be installed.  We need to
make sure the actual subs exist before we try to use them.

Author: haarg

lib/Moo/_Utils.pm

@@ -6,10 +6,14 @@ sub _getglob { \*{$_[0]} }
 sub _getstash { \%{"$_[0]::"} }
 
 use constant lt_5_8_3 => ( $] < 5.008003 or $ENV{MOO_TEST_PRE_583} ) ? 1 : 0;
-use constant can_haz_subutil => $INC{"Sub/Util.pm"}
-    || ( !$INC{"Sub/Name.pm"} && eval { require Sub::Util } );
-use constant can_haz_subname => $INC{"Sub/Name.pm"}
-    || ( !$INC{"Sub/Util.pm"} && eval { require Sub::Name } );
+use constant can_haz_subutil => (
+    $INC{"Sub/Util.pm"}
+    || ( !$INC{"Sub/Name.pm"} && eval { require Sub::Util } )
+  ) && defined &Sub::Util::set_subname;
+use constant can_haz_subname => (
+    $INC{"Sub/Name.pm"}
+    || ( !$INC{"Sub/Util.pm"} && eval { require Sub::Name } )
+  ) && defined &Sub::Name::subname;
 
 use Moo::_strictures;
 use Module::Runtime qw(use_package_optimistically module_notional_filename);

t/moo-utils-_name_coderef.t

@@ -2,8 +2,11 @@ use Moo::_strictures;
 use Test::More;
 
 BEGIN {
+  no warnings 'redefine';
   $INC{'Sub/Name.pm'} = 1;
+  defined &Sub::Name::subname or *Sub::Name::subname = sub {};
   $INC{'Sub/Util.pm'} = 1;
+  defined &Sub::Util::set_subname or *Sub::Util::set_subname = sub {};
 }
 
 use Moo::_Utils;