From @skids

$ perl6 -e 'sub f (​::T $a) { my T $b; { T.WHICH.say; $b.WHICH.say } }; f(1);'
Int
Int
$ perl6 -e 'sub f (​::T $a) { { my T $b; T.WHICH.say; $b.WHICH.say } }; f(1);'
Int
Method 'WHICH' not found for invocant of class 'T'
  in sub f at -e​:1
  in block <unit> at -e​:1

$ perl6 -e 'sub f (​::T $a) { my T $b; { T.say; $b.say } }; f(1);'
(Int)
(Int)

$ perl6 -e 'sub f (​::T $a) { { my T $b; T.say; $b.say } }; f(1);'
(Int)
Method 'say' not found for invocant of class 'T'
  in sub f at -e​:1
  in block <unit> at -e​:1

This is the root cause of commit 3d645fe4/7f9634764 breaking
:(​::T $a, T $b).perl

From @jnthn

On Thu Jul 02 22​:43​:18 2015, bri@​abrij.org wrote​:

$ perl6 -e 'sub f (​::T $a) { my T $b; { T.WHICH.say; $b.WHICH.say } };
f(1);'
Int
Int
$ perl6 -e 'sub f (​::T $a) { { my T $b; T.WHICH.say; $b.WHICH.say } };
f(1);'
Int
Method 'WHICH' not found for invocant of class 'T'
in sub f at -e​:1
in block <unit> at -e​:1

Fixed, and added test in S06-signature/type-captures.t.

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

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