[Backport] Overloading resolution: Handle SAM types more like Java and Scala 2 #12131
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Previously we caught this in Typer, but doing it in the `SAMType` extractor is more robust because it is used in overloading resolution, and this will be needed in a subsequent commit of this PR to avoid selecting the wrong overload in some cases. This required dropping the final flag from the compiler-generated ContextFunctionN, instead we explicitly prevent subclasses in RefChecks.
The two pass approach was originally introduced in 295c981 to lower the priority of implicit conversions, but just a month later in 6253125 a similar two pass approach was used to call resolveOverloaded which itself calls narrowByTrees, rendering the first commit moot. Therefore, we can just remove this first pass and all associated code without affecting anything.
`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.
odersky
approved these changes
Apr 17, 2021
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This backports #12097.