From @smls

The following fails to compile with "Undeclared routine​: regex"​:

  my token foo (&another-regex) { <&another-regex> };

Since routine arguments are supposed to become available to the routine body as lexical symbols, and <&foo> is the regex syntax for calling another regex by its lexical name, I assume this error is a bug.

For comparison, the following two variations do compile and work just fine​:

  my token foo ($another-regex) { $another-regex };

  my token foo (&another-regex) { {say &another-regex} };

(This is Rakudo version 2016.03-108-g98b5b8c built on MoarVM version 2016.03-104-g10d3971
implementing Perl 6.c.)

From @smls

UPDATE​:

It's not specific to the & sigil, as it also does not work with $ arguments​:

  my token foo ($a) { <$a> }; # Error​: Variable '$a' is not declared

And it's not specific to arguments, as it also does not work when the variable is declared using a '​:my' directive inside the regex body​:

  my token foo { :my &a = /a/; <&a> } # Error​: Undeclared routine​: a

On the other hand, it *does* work if the variable is declared in the parent scope​:

  my &a = /a/; my token foo { <&a> }

So the problems seems to be, that the < > regex call syntax looks up lexicals only in the *parent* scope of the regex it is used in, and not in the scope of the regex itself.

psch++ suggested that this may be intended behavior​: http://irclog.perlgeek.de/perl6/2016-04-11#i_12317275

psch++ also suggested a work-around by wrapping the regex in a Perl 6 method, so that the parameter becomes part of the parent scope of the regex​:

  my method foo (&a) { self.CALL_SUBRULE​: / <&a> / }

Still, it feels strange that the < > syntax behaves this way.
Needs confirmation from TimToady on the intended behavior.

From @smls

IRC comment <http://irclog.perlgeek.de/perl6/2016-04-22#i_12380115>&#8203;:

  23​:30 TimToady​: looks like a bug to me

From @zoffixznet

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> };␤»

@coke - Status changed from 'new' to 'open'

From @skids

On Sat, 09 Jul 2016 19​:48​:54 -0700, cpan@​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.