gistfile1.txt

sub trait_mod:<is>(Routine $r, :$symmetric!) {
    
    sub gen(::T2, ::T1) { 
        (sub (T1 $a, T2 $b) { say "woah"; $r($b, $a) }).instantiate_generic(nqp::hash('T1', T1, 'T2', T2)); 
    };
    
    my @params = $r.signature.params;
    die "symmetric only works on binary functions" unless @params == 2;
    gen(@params[0].type, @params[1].type);
    
}

multi sub foo(Int $a, Str $b) is symmetric {
    say $b x $a;
}

foo(10, "Hello"); # works
foo("Hello", 10); # results in
                  # Calling 'foo' will never work with argument types (str, int) (line 18)
                  #    Expected any of:
                  #    :(Int $a, Str $b)