Fix widening logic to keep instantiation within bounds #16417
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Before this commit, if `Config.checkConstraintsSatisfiable` was set to `true`,
then tests/pos/cls.scala would fail on `val b1` with:
Y is constrained to be >: Matchable but attempted to instantiate it to Int | String
This happens because when `widenInferred` widens a union, it constrains the
upper-bound of the parameter being instantiated to be a subtype of that union
(`tpw <:< bound`). This is usually fine, but if `isTransparent` is true, we'll
return the original unwidened type, which is now out of bounds.
To avoid this inconsistency, the `isTransparent` check is now moved before the
subtype check that updates the bounds.
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odersky
approved these changes
Nov 28, 2022
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Before this commit, if
Config.checkConstraintsSatisfiablewas set totrue, then tests/pos/cls.scala would fail onval b1with:This happens because when
widenInferredwidens a union, it constrains the upper-bound of the parameter being instantiated to be a subtype of that union (tpw <:< bound). This is usually fine, but ifisTransparentis true, we'll return the original unwidened type, which is now out of bounds.To avoid this inconsistency, the
isTransparentcheck is now moved before the subtype check that updates the bounds.