[perl #129160] enum key for optional positional does not work

[via RT]

# New Ticket Created by  Wenzel Peppmeyer 
# Please include the string:  [perl #129160]
# in the subject line of all future correspondence about this issue. 
# <URL: https://rt.perl.org/Ticket/Display.html?id=129160 >


enum Options(<Foo Bar>);
sub f(Foo $foo?){};
f();

# OUTPUT«Invocant requires an instance of type Int, but a type object was 
passed.  Did you forget a .new?␤  in sub f at <tmp> line 1␤  inblock <unit> at 
<tmp> line 1␤␤»

# expected to work
# 17:37 < TimToady> gfldex: please rakudobug that