From @masak

<masak> m​: role A​::B {}; sub foo(A $a) {}
<camelia> rakudo-moar 662447​: OUTPUT«===SORRY!=== Error while
compiling /tmp/TNiNkxUsdd␤Type A cannot be used as a nominal type on a
parameter [...]
* masak submits LTA rakudobug
<masak> it's true that A, which is in some sense less real than A​::B
(or not "inhabited" or whatever), but the error message should explain
that.
<masak> it's especially confusing that it calls A a "type".
<masak> in fact, I think it would be enough if it just said
"Containing package A cannot be used..."
<moritz> m​: package A { }; sub foo(A $a) { }
<camelia> rakudo-moar 662447​: OUTPUT«===SORRY!=== Error while
compiling /tmp/VqZ7Dwi6I3␤Type A cannot be used as a nominal type on a
parameter [...]
<moritz> masak​: not just containing packages
<masak> good point.
<masak> I mean, in some sense A is a type. in both cases.
<jnthn> It's not even "in some sense".
<masak> but the fact that it's not enough of a useful type is the more
important fact to highlight in the message.
<masak> it's not the kind of type to be used for typing :)
<jnthn> The message is already a LOT better than it just not accepting
anything at runtime.
<masak> granted.
<masak> m​: role A​::B {}; sub foo($a) { say $a }; foo(A)
<camelia> rakudo-moar 662447​: OUTPUT«(A)␤»
<masak> I mean, passing these things ain't a problem.
<jnthn> Just remove the word "Type" at the start of the error and it's
probably a lot less misleading.
<masak> +1
<jnthn> Or at least, less scizophrenic :)

From @masak

<masak> m​: role A​::B {}; sub foo(A $a) {}
<camelia> rakudo-moar 662447​: OUTPUT«===SORRY!=== Error while
compiling /tmp/TNiNkxUsdd␤Type A cannot be used as a nominal type on a
parameter [...]
* masak submits LTA rakudobug
<moritz> m​: package A { }; sub foo(A $a) { }
<camelia> rakudo-moar 662447​: OUTPUT«===SORRY!=== Error while
compiling /tmp/VqZ7Dwi6I3␤Type A cannot be used as a nominal type on a
parameter [...]
<masak> m​: role A​::B {}; my A $a; say $a
<camelia> rakudo-moar 662447​: OUTPUT«(A)␤»
<masak> m​: package A {}; my A $a; say $a
<camelia> rakudo-moar 662447​: OUTPUT«(A)␤»
<masak> why is this fine but the parameter case ain't?
<jnthn> masak​: Because that case is missing some checks, apparently... :P
* masak submits rakudobug
<jnthn> Probably something wants factoring out/re-using...
<masak> should the "package A" one be forbidden too?
<jnthn> Yes, it's the exact same case.
<jnthn> (As in, you can't differentiate the two in terms of the
objects being created)

From @usev6

Nowadays the error message no longer starts with the word 'Type'​:

$ perl6 -e 'role A​::B {}; sub foo(A $a) {}'
===SORRY!=== Error while compiling -e
A cannot be used as a nominal type on a parameter
at -e​:1
------> role A​::B {}; sub foo(A ⏏$a) {}

$ perl6 -e 'package A { }; sub foo(A $a) { }'
===SORRY!=== Error while compiling -e
A cannot be used as a nominal type on a parameter
at -e​:1
------> package A { }; sub foo(A ⏏$a) { }

The same error results with the following code, taken from ticket 117041​:

$ perl6 -e 'role A​::B {method foo(A $a) {} }'
===SORRY!=== Error while compiling -e
A cannot be used as a nominal type on a parameter
at -e​:1
------> role A​::B {method foo(A ⏏$a) {} }

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

From @usev6

Behaviour has changed and the latter two commands fail with the same error as the first two (for those compare ticket 123397, https://rt-archive.perl.org/perl6/Ticket/Display.html?id=123397):

$ /perl6 -e 'role A​::B {}; my A $a; say $a'
===SORRY!=== Error while compiling -e
A cannot be used as a nominal type on a variable
at -e​:1
------> role A​::B {}; my A $a⏏; say $a
  expecting any of​:
  constraint

$ perl6 -e 'package A {}; my A $a; say $a'
===SORRY!=== Error while compiling -e
A cannot be used as a nominal type on a variable
at -e​:1
------> package A {}; my A $a⏏; say $a
  expecting any of​:
  constraint

So, this works as expected now. I'm tagging the ticket 'testneeded'.

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

From @jnthn

On Wed Feb 11 11​:30​:22 2015, bartolin@​gmx.de wrote​:

Behaviour has changed and the latter two commands fail with the same
error as the first two (for those compare ticket 123397,
https://rt-archive.perl.org/perl6/Ticket/Display.html?id=123397):

$ /perl6 -e 'role A​::B {}; my A $a; say $a'
===SORRY!=== Error while compiling -e
A cannot be used as a nominal type on a variable
at -e​:1
------> role A​::B {}; my A $a⏏; say $a
expecting any of​:
constraint

$ perl6 -e 'package A {}; my A $a; say $a'
===SORRY!=== Error while compiling -e
A cannot be used as a nominal type on a variable
at -e​:1
------> package A {}; my A $a⏏; say $a
expecting any of​:
constraint

So, this works as expected now. I'm tagging the ticket 'testneeded'.

Added a typed exception for this (and the similar Parameter type error), and tested it.

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