Status update: It looks like STD's opinion on this has changed:
$ viv -c -e 'sub a (:$b($c)) {say $c}; a(:b)'
===SORRY!===
Subsignature not allowed after named parameter; please insert whitespace at
(eval) line 1:
--> sub a (:$b⏏($c)) {say $c}; a(:b)
Parse failed
Rakudo's error messages are
# New Ticket Created by "Carl Mäsak"
# Please include the string: [perl #120838]
# in the subject line of all future correspondence about this issue.
# https://rt.perl.org/Ticket/Display.html?id=120838 >
r: sub a (:$b) {say $b}; a(:b) # this works
rakudo-parrot 1049b4, rakudo-jvm 1049b4: O
