Compilation error due to composing an ineligible type
class X::Composition::NotComposable is Exception { }
Thrown when trying to compose a type into a target type, but the composer type cannot be used for composition (roles and enums are generally OK).
For example
class B { }
class C does B { }
dies with
===SORRY!=== B is not composable, so C cannot compose it
because does is reserved for role composition, and B is not a role, nor something that knows how to turn into a role.
The fix is to either make B a role, or use inheritance (class C is B { }) instead.
method target-name(--> Str:D)
Returns the name of the type that should be composed, but failed.
method composer(--> Mu)
Returns the type that should be composed into the target, but which isn't a role.