CC: Fix maximal capability handling and expand aliases#22341
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The subsumes check mistakenly allowed any capability to subsume `cap`, since `cap` is expanded as `caps.cap`, and by the path subcapturing rule `caps.cap <: caps`, where the capture set of `caps` is empty. This allowed quite a few hidden errors to go through. This commit fixes the subcapturing issue and all downstream issues caused by that fix. In particular: - Don't use path comparison for `x subsumes caps.cap` - Don't allow an opened existential on the left of a comparison to leak into a capture set on the right. This would give a "leak" error later in healCaptures. - Print pre-cc annotated capturing types with @retains annotations with `^`. The annotation is already rendered as a set in this case, but the `^` was missing. - Don't recheck `_` right hand sides of uninitialized variables. These were handled in ways that broke freshness checking. The new `uninitialied` scheme does not have this problem.
This is necessary because the compiler is free in previous phases to dealias or not. Therefore, capture checking should not depend on aliasing. The main difference is that now arguments to type aliases are not necessarily boxed. They are boxed only if they need boxing in the dealiased type.
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1. Fix various issues with maximal capabilities
The subsumes check mistakenly allowed any capability to subsume
cap, sincecapis expandedas
caps.cap, and by the path subcapturing rulecaps.cap <: caps, where the capture set ofcapsis empty. This allowed quite a few hidden errors to go through. This commit fixes thesubcapturing issue and all downstream issues caused by that fix.
2. Expand aliases when mapping explicit types in Setup
This is necessary because the compiler is free in previous phases to dealias or not.
Therefore, capture checking should not depend on aliasing. The main difference is that
now arguments to type aliases are not necessarily boxed. They are boxed only if they need
boxing in the dealiased type.