Backport "Make overload pruning based on result types less aggressive" to 3.3 LTS #265
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`adaptByResult` was introduced in 2015 in 54835b6 as a last step in overloading resolution: > Take expected result type into account more often for overloading resolution > > Previously, the expected result type of a FunProto type was ignored and taken into > account only in case of ambiguities. arrayclone-new.scala shows that this is not enough. > In a case like > > val x: Array[Byte] = Array(1, 2) > > we typed 1, 2 to be Int, so overloading resulution would give the Array.apply of > type (Int, Int*)Array[Int]. But that's a dead end, since Array[Int] is not a subtype > of Array[Byte]. > > This commit proposes the following modified rule for overloading resulution: > > A method alternative is applicable if ... (as before), and if its result type > is copmpatible with the expected type of the method application. > > The commit does not pre-select alternatives based on comparing with the expected > result type. I tried that but it slowed down typechecking by a factor of at least 4. > Instead, we proceed as usual, ignoring the result type except in case of > ambiguities, but check whether the result of overloading resolution has a > compatible result type. If that's not the case, we filter all alternatives > for result type compatibility and try again. In i21410.scala this means we end up checking: F[?U] <:< Int (where ?U is unconstrained, because the check is done without looking at the argument types) The problem is that the subtype check returning false does not mean that there is no instantiation of `?U` that would make this check return true, just that type inference was not able to come up with one. This could happen for any number of reason but commonly will happen with match types since inference cannot do much with them. We cannot avoid this by taking the argument types into account, because this logic was added precisely to handle cases where the argument types mislead you because adaptation isn't taken into account. Instead, we can approximate type variables in the result type to trade false negatives for false positives which should be less problematic here. Fixes scala#21410. [Cherry-picked 528d0f0]
Overloading may create a temporary symbol via `Applications#resolveMapped`, but before this commit, this symbol did not carry the annotations from the original symbol. This meant in particular that `isInlineable` would always return false for them. This matters because during the course of overloading resolution we might call `ProtoTypes.Compatibility#constrainResult` which special-cases transparent inline methods. Fixes a regression in Monocle introduced in the previous commit. wip [Cherry-picked f7259cf]
The changes two commits ago were not enough to handle i21410b.scala because we
end up checking:
Tuple.Map[WildcardType(...), List] <: (List[Int], List[String])
which fails because a match type with a wildcard argument apparently only gets
reduced when the match type case is not parameterized.
To handle this more generally we use AvoidWildcardsMap to remove wildcards from
the result type, but since we want to prevent false negatives we start with
`variance = -1` to get a lower-bound instead of an upper-bound.
[Cherry-picked 32ac2e6]
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I don't know the context, but I remember "helping" with similar |
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The second one is already in the LTS branch, the first one is currently being back ported 😄 |
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No regressions detected in the community build up to backport-lts-3.3-21744. |
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Backports scala#21744 to the 3.3.7.
PR submitted by the release tooling.