From rightfold@gmail.com

Consider the following code​:

use v6.c;

subset T of List where *[0] eqv 1;

class R {
  multi method f(T​:D $xs) { self.f(42) }
  multi method f(Any​:D $xs) { say $xs }
}

R.f([1, 2]);
R.f([2, 2]);

It fails with the following output​:

42
Type check failed in binding $xs; expected T but got Array
  in method f at /tmp/foo.pm6 line 6
  in block <unit> at /tmp/foo.pm6 line 11

However, when the R.f([1, 2]) call is removed, the program succeeds.

Thanks to masak++ for helping minimizing the example.

Rakudo version​:

This is Rakudo version 2015.12 built on MoarVM version 2015.12 implementing Perl 6.c.

From @masak

<ely-se> the interesting thing is that Any​:D is a catchall
<masak> ely-se​: if I remove the first (successful) call to .f, the second one starts working!
<masak> ely-se​: feels like a screwing-up of method caches...
<ely-se> eek caches
<masak> aye
<masak> ely-se​: inlining T as a `where` in the siggie also makes things work properly
<masak> ely-se​: (so one way to work around it would be to inline the subtype)
<masak> ely-se​: as far as I can see, it fails because we first match T correctly, but in the second invocation we match List but not T.
<ely-se> yay I inlined the constraints and now my tests pass :)
<masak> m​: subset T of List where { .[0] == 1 }; .f([1, 2]) && .f([2, 2]) given class C { multi method f(T​:D $) { self.f(42) }; multi method f($xs) { say $xs } }
<camelia> rakudo-moar d67cb0​: OUTPUT«42␤Type check failed in binding <anon>; expected T but got Array␤ in method f [...]
<masak> getting golfier by the minute
<masak> m​: subset N of Int where * == 5; .f(5), .f(7) given class C { multi method f(N​:D $) { self.f("OH HAI") }; multi method f($x) { say $x } }
<camelia> rakudo-moar d67cb0​: OUTPUT«OH HAI␤Type check failed in binding <anon>; expected N but got Int␤ in method f [...]
<masak> ely-se​: it's not just Lists

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

From @LLFourn

I just hit this. Another way of getting the example code working is to
remove the '​:D'.

use v6.c;

subset T of List where *[0] eqv 1;

class R {
  multi method f(T $xs) { self.f(42) }
  multi method f(Any​:D $xs) { say $xs }
}

R.f([1, 2]);
R.f([2, 2]);

# ^^ works fine
So something to do with Definite types and caching i'd say.

From @zoffixznet

Thanks for the report.

This has been fixed in rakudo/rakudo@68afa3ff
Tests added in Raku/roast@b00a4f4a5c

--
Cheers,
ZZ | https://twitter.com/zoffix

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