Type inference not working with java overloads

[eatkins]

Forgive me if this is a duplicate, but there are cases where the scala compiler will infer Nothing where it is unexpected. This came up in playframework/playframework#10442. I was able to reproduce with 2.10.7, 2.11.11, 2.12.12 and 2.13.3.

reproduction steps

create a java file Foo.java:

public class Foo {
  void foo(Foo foo) {}
  void foo(java.util.Optional<Foo> foo) {}
}

create a scala file Bar.scala:

object Bar {
  Foo.foo(java.util.Optional.empty())
}

compile the project

problem

sbt:overload> compile
[info] Compiling 1 Scala source and 1 Java source to /Users/ethanatkins/work/scratch/overload/target/scala-2.12/classes ...
[error] /Users/ethanatkins/work/scratch/overload/src/main/scala/Bar.scala:2:7: overloaded method value foo with alternatives:
[error]   (foo: java.util.Optional[Foo])Unit <and>
[error]   (foo: Foo)Unit
[error]  cannot be applied to (java.util.Optional[Nothing])
[error]   Foo.foo(java.util.Optional.empty());
[error]       ^
[error] one error found
[error] (Compile / compileIncremental) Compilation failed
[error] Total time: 1 s, completed Sep 4, 2020 8:10:45 AM

If I remove the overloaded foo method that takes a Foo, it does compile.

[som-snytt]

This is a known limitation and there are similar tickets which ticket fatigue prevents me from discovering.

Worth noting that if the overloads share the parameter of interest, inference is improved:

        public void foo(java.util.Optional<Foo> foo, Foo actual) {}
        public void foo(java.util.Optional<Foo> foo) {}

then

Bar.this.x.foo(java.util.Optional.empty[t12139.Foo]())   // not Nothing

The spec stipulates that arguments are typechecked without an expected type, but does not preclude adaptations after an overload is selected.

[eatkins]

I'm not surprised to hear this is a known limitation. Feel free to close this if it's covered elsewhere.

[SethTisue]

Duplicate of #10542 and/or #10046? (At least, they look similar, offhand.)

[SethTisue]

We can reopen if someone can substantiate that this one is actually different.

[som-snytt]

Seth is immune to ticket fatigue. Scala Center ought to present the Seth Tissue Award in Issuology in recognition of exceptional cross-ticketing.