The check for implemented stub method do not find methods generated by a attribute handles

[FCO]

The Problem

$ perl6 -e 'role R { method r {...} }; class C does R { has R $.a handles <r> }'
===SORRY!=== Error while compiling -e
Method 'r' must be implemented by C because it is required by roles: R.
at -e:1

I was searching for the reason, and looks that the compose_attributes (that creates the handled methods) is called after the check of the roles' stubed methods.

Expected Behavior

That should work

Actual Behavior

breaks at compile time sayng that the method wasnt implemented

Steps to Reproduce

just run the one-liner

Environment

• Operating system: ubuntu and mac os x
• Compiler version (perl6 -v):

This is Rakudo version 2018.08 built on MoarVM version 2018.08
implementing Perl 6.c.

[jnthn]

It's a (I think solvable) ordering and factoring problem.

Attribute composition must happen after role composition for multiple reasons, so it's not a simple case of "re-order the steps". That means we have to break out the check for missing requirements - which I think happens inside the role composition code at the moment - so that it can happen after attribute composition. And then somehow communicate this between the phases. So, a bit less neat, but probably worth it, since folks stub their toe on this every now and then.