is Type fails in parameters

[alabamenhu]

The Problem

The trait form of specifying types does not work properly on parameters. This is particular problematic for containers as their container (or key, in the case of Associative) type cannot be specified any other way but for a clunky work around using multi subs.

Expected Behavior

sub foo(%s is Set) {  
    say %s.WHAT; # Set (or some subclass thereof)
}

Actual Behavior

sub foo(%s is Set) {  # compile time error, see output
    say %s.WHAT; 
}

Output

Can't use unknown trait 'is' -> 'unknown subtype' in a parameter declaration.
at -e:1
    expecting any of:
        rw
        readonly
        copy
        required
        raw
        leading_docs
        trailing_docs

Environment

All versions of Rakudo, error message changed in early 2019. See bisect output

[lizmat]

FWIW, I think the workaround of sub foo(Set $s) { } is not really an inconvenience?

[alabamenhu]

FWIW, I think the workaround of sub foo(Set $s) { } is not really an inconvenience?

I mean sure, for sets. But we have sigils and I like to use them :) Also, assignment behavior is affected if someone were to want to assign into an associative/positional parameter.