Freeze constraints while calculating GADT full bounds #18222
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While studying a bug, I discovered that the code I'd written to avoid bidirectionalily in GADT fullBounds was inadvertently recording GADT constraints, assisted by the fact that, when running without -Yno-deep-subtypes, TypeComparer's escape hatch for deep recursions avoids it crashing. For instance a GADT symbol was compared >: Nothing and thus recorded as having Nothing as a lower bound. So I added freezing, and while I was at it I updated the docs I hadn't changed last time, and I made the subtyping check in the only direction necessary.
nicolasstucki
approved these changes
Jul 20, 2023
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While studying a bug, I discovered that the code I'd written to avoid
bidirectionalily in GADT fullBounds was inadvertently recording GADT
constraints, assisted by the fact that, when running without
-Yno-deep-subtypes, TypeComparer's escape hatch for deep recursions
avoids it crashing. For instance a GADT symbol was compared >: Nothing
and thus recorded as having Nothing as a lower bound.
So I added freezing, and while I was at it I updated the docs I hadn't
changed last time, and I made the subtyping check in the only direction
necessary.