Avoid using Ranges in exact substitutions
#17532
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This improves the completeness of capture checking in the following case:
```scala
def run(io: IO^)(op: Op^{io}): Unit = ???
val myIO: IO^ = ???
val myOp: Op^{myIO} = ???
run(myIO)(myOp)
```
The last line previously won't work because when substitute the parameter `io`
to `myIO` in the type `(op: Op^{io}): Unit` the type `Op^{io}` get approximated
into `Nothing` contravariantly. This is an overshot since the substitution [io
:= myIO] is exact (or precise) and the approximation (or avoidance) is
unnecessary.
This commit resolves this issue by checking whether the `to` only contains
`CaptureRef` in `SubstParamsMap`. If this condition is met, then it is an exact
substitution and we can apply it on the capture set without approximations.
odersky
approved these changes
May 22, 2023
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This improves the completeness of capture checking in the following case:
The last line previously won't work because when substitute the parameter
iotomyIOin the type(op: Op^{io}): Unitthe typeOp^{io}get approximated intoNothingcontravariantly. This is an overshot since the substitution [io := myIO] is exact (or precise) and the approximation (or avoidance) is unnecessary.This commit resolves this issue by checking whether the
toonly containsCaptureRefinSubstParamsMap. If this condition is met, then it is an exact substitution and we can apply it on the capture set without approximations.