Improve GADT reasoning for pattern alternatives #23205
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Previously it fails, but now it is ok
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Pull Request Overview
This PR refines GADT constraint handling for pattern alternatives by collecting constraints from each case and using their common, necessary constraint instead of discarding all GADT reasoning.
- Collects GADT constraints per alternative in
Typer.scalaand restores the weakest common constraint if it exists. - Adds
necessaryGadtConstraintinTypeComparer.scalato compute a constraint subsumed by all alternatives. - Updates existing positive and negative tests to reflect the new behavior.
Reviewed Changes
Copilot reviewed 9 out of 9 changed files in this pull request and generated 1 comment.
| File | Description |
|---|---|
| tests/pos/gadt-alt-doc1.scala | Adds a positive test demonstrating a disallowed alternative now errors. |
| tests/neg/gadt-alternatives.scala & various gadt-alt-expr*.scala | Updates negative tests to expect new accept/reject behavior. |
| compiler/src/dotty/tools/dotc/typer/Typer.scala | Collects and aggregates GADT constraints instead of discarding them. |
| compiler/src/dotty/tools/dotc/core/TypeComparer.scala | Introduces necessaryGadtConstraint and related import updates. |
Comments suppressed due to low confidence (1)
compiler/src/dotty/tools/dotc/core/TypeComparer.scala:28
- [nitpick] The combined import
import scala.util.boundary, boundary.breakmay importboundary.breakfrom the wrong package; preferimport scala.util.boundary.breakor separate the imports to ensure the correct symbol is brought into scope.
import scala.util.boundary, boundary.break
| val trees1 = tree.trees.mapconserve: t => | ||
| nestedCtx.gadtState.restore(preGadt) | ||
| val res = typed(t, pt)(using nestedCtx) | ||
| gadtConstrs += ctx.gadt |
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Recording ctx.gadt likely captures the outer context rather than the updated nested context; it should use nestedCtx.gadt to collect the actual constraints produced by each alternative.
Suggested change
| gadtConstrs += ctx.gadt | |
| gadtConstrs += nestedCtx.gadt |
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fixes #22882.
Previously, for a pattern alternative:
we ignore GADT constraints derived from it, which is too restrictive. Now we try to find a GADT constraint that is subsumed by all and use it, and only ignore GADT constraints when such a constraint cannot be found.