Fix #4884: asSeenFrom on incorrect prefix in type applications #4900
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I came across this problem independently when working on transparent functions. It's quite a subtle problem. Afters several false starts I found a fix which looks right in #4902. |
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In this code we fetch the type parameter bounds from the definition, so we must substitute their definition using `asSeenFrom` with the correct prefix. The correct prefix is however hard to find: `TypeApplications.typeParams` finds a `self: TypeRefs` (after however many steps), the correct prefix is `self.prefix`. In fact, a compositional typing rule would obtain the bounds from the type (or, well, kind) of the type constructor; that would avoid the need for using `asSeenFrom`. We only have a way to obtain such a kind for `NamedType` (where we can take the `info`). However, that seems enough.
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Fair enough — my original fix works for |
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WIP fix #4884. PR for testing. Currently causing a stackoverflow on a benchmark, but only with the non-bootstrapped compiler; what's worse is that the stackoverflow doesn't go away when increasing stack size.
Approach
In this code we fetch the type parameter bounds from the definition, so we must
substitute their definition using
asSeenFromwith the correct prefix.The correct prefix is however hard to find:
TypeApplications.typeParamsfindsa
self: TypeRefs(after however many steps), the correct prefix isself.prefix.In fact, a compositional typing rule would obtain the bounds from the type (or,
well, kind) of the type constructor; that would avoid the need for using
asSeenFrom. We only have a way to obtain such a kind forNamedType(where wecan take the
info). That might be enough, but there might be another way tofix this.