From bobn@ieee.org

This works OK (copied from https://docs.perl6.org/language/list#Typing):
  perl6 -e 'sub mean(Int @​a) {@​a.sum / @​a.elems} ; my Int @​b = 1, 3,
5; say mean(@​b);'

These do not​:
  % perl6 -e 'sub mean(UInt @​a) {@​a.sum / @​a.elems} ; my UInt @​b = 1,
3, 5; say mean(@​b);'
  Constraint type check failed for parameter '@​a'
  in sub mean at -e line 1
  in block <unit> at -e line 1
  %
  % perl6 -e 'subset Data of UInt where * < 2**32 ; sub mean(Data @​a)
{@​a.sum / @​a.elems} ; my Data @​b = 1, 3, 5; say mean(@​b);'
  Constraint type check failed for parameter '@​a'
  in sub mean at -e line 1
  in block <unit> at -e line 1
  %

Environment​:
  % perl6 --version
  This is Rakudo version 2016.06 built on MoarVM version 2016.06
  implementing Perl 6.c.
  % uname -a
  Linux host 4.7.0-1-amd64 #​1 SMP Debian 4.7.5-1 (2016-09-26) x86_64
GNU/Linux
  % dpkg-ck rakudo
  Desired=Unknown/Install/Remove/Purge/Hold
  |
Status=Not/Inst/Conf-files/Unpacked/halF-conf/Half-inst/trig-aWait/Trig-pend
  |/ Err?=(none)/Reinst-required (Status,Err​: uppercase=bad)
  ||/ Name Version
+++-================================================-===========================
  ii rakudo 2016.06-1
  ii rakudo-lib 2016.06-1

From @zoffixznet

Slightly golfed on 2016.09-200-g2c681c5​:

$ ./perl6 -e 'sub (Int @​) {}(my Int @​b)'
$ ./perl6 -e 'sub (Int @​) {}(my UInt @​b)'
$ ./perl6 -e 'sub (UInt @​) {}(my UInt @​b)'
Constraint type check failed for parameter '<anon>'
  in sub at -e line 1
  in block <unit> at -e line 1

From @zoffixznet

And subsets that don't involve UInt seem to work fine​:

$ ./perl6 -e 'subset Foo; sub (Foo @​) {}(my UInt @​b)'
$ ./perl6 -e 'subset Foo; sub (Foo @​) {}(my Foo @​b)'

From 1parrota@gmail.com

FWIW, that's true on 2016.07.01 as well. build-date = 2016-07-22T12​:37​:00Z

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

From ctp@fastmail.co.uk

Note the type must have a constraint​:

$ perl6 -e 'subset Foo of Int where * > 1; sub f(Foo @​i){}; f(my Foo @​a);'
Constraint type check failed for parameter '@​i'
  in sub f at -e line 1
  in block <unit> at -e line 1