[CS] Restore a type variable for compatibility with rdar://85263844 #40224
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Despite being otherwise disconnected from the constraint system, it's possible for it to affect how we type-check tuple matches in certain cases. This is due to the fact that: - It can have a lower type variable ID than an opened generic parameter type, so becomes the representative when merged with it. And because it has a different locator, this can influence binding prioritization. - Tuple subtyping is broken, as it's currently a *weaker* relationship than conversion. Therefore, temporarily restore this bit of logic for language versions < 6. If possible, we should try and fix tuple subtying in Swift 6 mode to not accept label mismatches, so that it's not more permissive than tuple conversion. rdar://85263844
This is something that we'd like to fix to bring in line with tuple conversion, so start warning on cases where it occurs.
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@swift-ci please test |
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@swift-ci please test source compatibility |
xedin
approved these changes
Nov 17, 2021
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This bit of logic was removed in #39543. However it turns out that despite this type variable being otherwise disconnected from the constraint system, it's possible for it to affect how we type-check tuple matches in certain cases.
This is due to the fact that:
Therefore, temporarily restore this bit of logic for language versions < 6. In addition, start warning on cases where we currently permit tuple label mismatches in subtyping. If possible, we should try and fix tuple subtying in Swift 6 mode to not accept label mismatches, so that it's not more permissive than tuple conversion.
rdar://85263844