[perl #127872] <&foo> [REGEX] syntax does not find &foo in the regex's own scope, only in the parent scope

[Brian S. Julin via RT]

On Sat, 09 Jul 2016 19:48:54 -0700, c...@zoffix.com wrote:
> Still present in rakudo 405519
> 
> <Zoffix> m: my token foo ($a) { <$a> };
> <camelia> rakudo-moar 405519: OUTPUT«===SORRY!=== Error while
> compiling <tmp>␤Variable '$a' is not declared␤at <tmp>:1␤------> my
> token foo ($a) { <⏏$a> };␤»


In addition to compiling, actually calling the regex presents some additional
restrictions:

To use arguments with the <&a()> form, you cannot use a sub or method:

$ perl6 -e 'my method a () { "a" ~~ /aa/ }; my token foo { <&a()> }; say "aa" 
~~ /<foo>/'
Cannot look up attributes in a Nil type object
  in regex foo at -e line 1
  in block <unit> at -e line 1

...but regexes work fine.

To use the <&a> form, you merely need to define it in the parent scope:

$ perl6 -e 'my token foo { :my regex a { a }; <&a> }; "aa" ~~ /<foo>/'
===SORRY!=== Error while compiling -e
Undeclared routine:
    a used at line 1
$ perl6 -e 'my regex a { a }; my token foo { <&a> }; say "aa" ~~ /<foo>/'
｢a｣
 foo => ｢a｣
$ perl6 -e 'my token a { a }; my token foo { <&a> }; say "aa" ~~ /<foo>/'
｢a｣
 foo => ｢a｣
$ perl6 -e 'my sub a ($cursor) { "aa" ~~ /a/ }; my token foo { <&a> }; say "aa" 
~~ /<foo>/'
｢a｣
 foo => ｢a｣
$ perl6 -e 'my method a ($cursor:) { "aa" ~~ /a/ }; my token foo { <&a> }; say 
"aa" ~~ /<foo>/'
｢a｣
 foo => ｢a｣

...

...however to use the <a> form and hit a lexical method, it can be directly in 
the scope:

$ perl6 -e 'my token foo { :my regex a { a }; <a> }; say "aa" ~~ /<foo>/'
｢a｣
 foo => ｢a｣
  a => ｢a｣
bri@atlas:~/git/roast$ perl6 -e 'my regex a { a }; my token foo { <a> }; say 
"aa" ~~ /<foo>/'
｢a｣
 foo => ｢a｣
  a => ｢a｣

...but is must be a regex, not a sub or method

$ perl6 -e 'my method a ($cursor:) { "aa" ~~ /a/ }; my token foo { <a> }; say 
"aa" ~~ /<foo>/'
No such method 'a' for invocant of type 'Match'. Did you mean 'at'?
  in regex foo at -e line 1
  in block <unit> at -e line 1

$ perl6 -e 'my sub a ($cursor) { "aa" ~~ /a/ }; my token foo { <a> }; say "aa" 
~~ /<foo>/'
No such method 'a' for invocant of type 'Match'. Did you mean 'at'?
  in regex foo at -e line 1
  in block <unit> at -e line 1

... that latter part may be to spec.  However the spec, after being very 
careful to only
talk about regexes when describing <a>'s lexical predelictions, if this is 
indeed intentional,
slips up by dropping the word "routine" in the last paragraph of that section.  
A bit
of spec clarification would help there.

Internally, <&a> seems to route through assertion:sym<var> which installs a 
CALL_SUBRULE invocation,
and <a> routes through assertion:sym<name> which tacks on a '&' sigil, perfoms 
a $*W.regex_in_scope
check, verifies that there was no dot preceding it, and then creates a new 
lexical var of the same
sigiled name, but does not use a CALL_SUBRULE instruction... just sets up RX 
QAST nodes.

The tests in S05-metasyntax/angle-brackets.t auto-RTed as RT#124519 through 
RT#124523 make a lot
of wrong assumptions about what the return value is and what gets done with it.