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[Backport] Overloading resolution: Handle SAM types more like Java and Scala 2 #12131

Merged
merged 3 commits into from
Apr 17, 2021
Merged

[Backport] Overloading resolution: Handle SAM types more like Java and Scala 2 #12131

merged 3 commits into from
Apr 17, 2021

Commits on Apr 17, 2021

  1. SAM types cannot be sealed or final 

    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.
    @smarter
    smarter committed Apr 17, 2021
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  2. Drop first pass in narrowByTrees 

    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.
    @smarter
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  3. Overloading resolution: Handle SAM types more like Java and Scala 2 

    `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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