From zefram@fysh.org

$ ./perl6 -e 'sub aa (Mu​:D $a) { }; aa(Int)'
Parameter '$a' requires an instance of type Int, but a type object was passed. Did you forget a .new?
  in sub aa at -e​:1
  in block <unit> at -e​:1
$ ./perl6 -e 'sub aa (Mu​:U $a) { }; aa(3)'
Parameter '$a' requires a type object of type Int, but an object instance was passed. Did you forget a 'multi'?
  in sub aa at -e​:1
  in block <unit> at -e​:1

In both cases it is correct to get an error of this general type,
regarding the definedness constraint. But the desired type is
misreported​: the class of the argument passed, rather than the desired
class, is being stated.

-zefram

From @zoffixznet

Fixed in rakudo/rakudo#845

Tests added in Raku/roast@c0feebdbb1

The RT System itself - Status changed from 'new' to 'open'

From @usev6

With Rakudo commit we get the same output on MoarVM and JVM​:

$ ./perl6-j -e 'sub aa (Mu​:D $a) { }; aa(Int)'
Parameter '$a' requires an instance of type Mu, but a type object was passed. Did you forget a .new?
  in sub aa at -e line 1
  in block <unit> at -e line 1

$ ./perl6-j -e 'sub aa (Mu​:U $a) { }; aa(3)'
Parameter '$a' requires a type object of type Mu, but an object instance was passed. Did you forget a 'multi'?
  in sub aa at -e line 1
  in block <unit> at -e line 1

I'm closing this ticket as 'resolved'.

@usev6 - Status changed from 'open' to 'resolved'