The underlying bug in Rakudo I spoke about is now fixed (in the 'glr' branch). There is a test in roast -- cmp. Ticket 124079 (https://rt.perl.org/Ticket/Display.html?id=124079). The shortened code from the bug report dies as expected:
$ perl6 -e 'my Str @uri = ([<ldap1>], [<ldap2>]); say "alive"' Type check failed in assignment to '@uri'; expected 'Str' but got 'Array' in block <unit> at -e:1 I'm closing this ticket as 'resolved'.