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Different overloading rules in Scala 2 and 3 for SAMs #11938
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Here's the comment that backs my analysis: /** This private version of `resolveOverloaded` does the bulk of the work of
* overloading resolution, but does neither result adaptation nor apply insertion.
* It might be called twice from the public `resolveOverloaded` method, once with
* implicits and SAM conversions enabled, and once without.
*/
private def resolveOverloaded1(alts: List[TermRef], pt: Type)(using Context): List[TermRef] = ... |
In fact Applications.scala shows several comments and examples referring to SAM types (just grep for SAM). It shows that there are difficult tradeoffs to make. |
odersky
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Always consider a function type A as conforming to a corresponding SAM type B, unless B is a subtype of A. The unless... part is so that `PartialFunction[A, B]` is seen as strictly better than `A => B`. This is an alternate fix for scala#11938 that does not treat FunctionalInterfaces specifically.
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Always consider a function type A as conforming to a corresponding SAM type B, unless B is a subtype of A. The unless... part is so that `PartialFunction[A, B]` is seen as strictly better than `A => B`. This is an alternate fix for scala#11938 that does not treat FunctionalInterfaces specifically.
smarter
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`resolveOverloaded` is called twice, first with implicit conversions off, then on. Before this commit, turning off implicit conversions also turned off SAM conversions, this behavior does not match Java or Scala 2 which means we could end up picking a different overload than they do (cf scala#11938). This commit enables SAM conversions in the first pass, _except_ for conversions to PartialFunction as that would break a lot of existing code and wouldn't match how either Java or Scala 2 pick overloads. This is an alternative to scala#11945 (which special-cased SAM types with an explicit `@FunctionalInterfaces` annotation) and scala#11990 (which special-cased SAM types that subtype a scala Function type). Special-casing PartialFunction instead seems more defensible since it's already special-cased in Scala 2 and is not considered a SAM by Java. Fixes scala#11938.
smarter
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`resolveOverloaded` is called twice, first with implicit conversions off, then on. Before this commit, turning off implicit conversions also turned off SAM conversions, this behavior does not match Java or Scala 2 which means we could end up picking a different overload than they do (cf scala#11938). This commit enables SAM conversions in the first pass, _except_ for conversions to PartialFunction as that would break a lot of existing code and wouldn't match how either Java or Scala 2 pick overloads. This is an alternative to scala#11945 (which special-cased SAM types with an explicit `@FunctionalInterfaces` annotation) and scala#11990 (which special-cased SAM types that subtype a scala Function type). Special-casing PartialFunction instead seems more defensible since it's already special-cased in Scala 2 and is not considered a SAM type by Java. Fixes scala#11938.
smarter
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`resolveOverloaded` is called twice, first with implicit conversions off, then on. Before this commit, turning off implicit conversions also turned off SAM conversions, this behavior does not match Java or Scala 2 which means we could end up picking a different overload than they do (cf scala#11938). This commit enables SAM conversions in the first pass, _except_ for conversions to PartialFunction as that would break a lot of existing code and wouldn't match how either Java or Scala 2 pick overloads. This is an alternative to scala#11945 (which special-cased SAM types with an explicit `@FunctionalInterfaces` annotation) and scala#11990 (which special-cased SAM types that subtype a scala Function type). Special-casing PartialFunction instead seems more defensible since it's already special-cased in Scala 2 and is not considered a SAM type by Java. Fixes scala#11938.
smarter
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Apr 17, 2021
`resolveOverloaded` is called twice, first with implicit conversions off, then on. Before this commit, turning off implicit conversions also turned off SAM conversions, this behavior does not match Java or Scala 2 which means we could end up picking a different overload than they do (cf scala#11938). This commit enables SAM conversions in the first pass, _except_ for conversions to PartialFunction as that would break a lot of existing code and wouldn't match how either Java or Scala 2 pick overloads. This is an alternative to scala#11945 (which special-cased SAM types with an explicit `@FunctionalInterfaces` annotation) and scala#11990 (which special-cased SAM types that subtype a scala Function type). Special-casing PartialFunction instead seems more defensible since it's already special-cased in Scala 2 and is not considered a SAM type by Java. Fixes scala#11938.
michelou
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`resolveOverloaded` is called twice, first with implicit conversions off, then on. Before this commit, turning off implicit conversions also turned off SAM conversions, this behavior does not match Java or Scala 2 which means we could end up picking a different overload than they do (cf scala#11938). This commit enables SAM conversions in the first pass, _except_ for conversions to PartialFunction as that would break a lot of existing code and wouldn't match how either Java or Scala 2 pick overloads. This is an alternative to scala#11945 (which special-cased SAM types with an explicit `@FunctionalInterfaces` annotation) and scala#11990 (which special-cased SAM types that subtype a scala Function type). Special-casing PartialFunction instead seems more defensible since it's already special-cased in Scala 2 and is not considered a SAM type by Java. Fixes scala#11938.
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Related to #11899:
If I compile and run this code with one version of
map
I get 42. But if I uncomment the first generic overload ofmap
I getTest$$$Lambda$2/1705736037@12bc6874
in Scala 3. Scala 2 still prints 42.The difference seems to be that SAM conversion is considered only in second rank for eligibility of overloading resolution. So Scala 3 picks the first overload, but Scala 2 picks the second. The problem in #11899 seems to be that Scala-2 thought it could do a SAM conversion when doing overloading resolution but ran into a type error later.
Interestingly, if the second overload of
map
forString
is uncommented, but the first is left commented, both Scala 2 and 3 print 42. So Scala 3 does figure out it can do a SAM conversion if no other alternative is applicable.The text was updated successfully, but these errors were encountered: