Fix #3246: Refine handling of postfix _ for non-functions

[odersky]

No description provided.

[odersky]

@allanrenucci All patterns you showed in #3246 should now be handled without error under -language:Scala2

[allanrenucci]

I'll try to revert the workaround in scalatest and see if I can compile with this patch

[allanrenucci]

class Test {
  def foo(fun: () => Unit) = ???
  def bar(fun: => Any) = foo(fun _)
}

This test case still does not compile with Dotty

[odersky]

Here's the error message I get (btw, for future reference, it would be good to add the actual error message to an issue or comment).

11 |  def bar(fun: => Any) = foo(fun _)
   |                             ^^^
   |                             found:    () => Any
   |                             required: () => Unit

This looks OK to me. Why should it compile? I don't see how fun _ could expand to anything but a () => Any. I think this is a bug in scalac.

[allanrenucci]

I agree it shouldn't compile. But then, should the following code compile?

class Test {
  def foo(fun: () => Unit) = ???
  def bar(fun: => Any) = foo(() => fun)
}

[allanrenucci]

Can be a val now

[allanrenucci]

As a side effect of this PR, we now accept this code under -language:Scala2 (scalac rejects it)

class Test {
  def foo(x: Int) = 1
  val bar: () => Int = foo(1) _
}

It is definitely an improvement as we used to emit a type error:

-- [E007] Type Mismatch Error: tests/allan/Test.scala:3:26 ---------------------
3 |  val bar: () => Int = foo(1) _
  |                       ^^^^^^
  |                       found:    Int
  |                       required: () => Int
  |                       

one error found

Now:

-- Migration Warning: tests/allan/Test.scala:3:30 ------------------------------
3 |  val bar: () => Int = foo(1) _
  |                       ^^^^^^^^
  |                       not a function: foo(1); cannot be followed by `_'

[allanrenucci]

class Test {
  def foo = 1
  val bar: () => Int = foo _
}

-- Migration Warning: tests/allan/Test.scala:3:27 ------------------------------
3 |  val bar: () => Int = foo _
  |                       ^^^^^
  |                       not a function: foo; cannot be followed by `_'

I think the warning is misleading in this case. If foo is defined as def foo(), then no warning will be emitted at all, even though the _ is not needed. I think we should depreciate _ in all cases.

[allanrenucci]

class Test {
  def foo(x: Int) = 1
  val bar: () => Int = foo _
}

This now crashes under -language:Scala2

> dotc -d out tests/allan/Test.scala -language:Scala2 -migration
[info] Running dotty.tools.dotc.Main -d out tests/allan/Test.scala -language:Scala2 -migration -classpath /Users/renucci/Documents/Projects/dotty/library/target/scala-2.12/dotty-library_2.12-0.4.0-bin-SNAPSHOT-nonbootstrapped.jar
-- Migration Warning: tests/allan/Test.scala:3:27 ------------------------------
3 |  val bar: () => Int = foo _
  |                       ^^^^^
  |                       not a function: foo; cannot be followed by `_'

exception occurred while typechecking tests/allan/Test.scala

exception occurred while compiling tests/allan/Test.scala
Exception in thread "main" java.lang.AssertionError: assertion failed
	at scala.Predef$.assert(Predef.scala:204)
	at dotty.tools.dotc.ast.Trees$Tree.withType(Trees.scala:121)
	at dotty.tools.dotc.typer.Typer.$anonfun$typedTypeTree$1(Typer.scala:1071)
	at dotty.tools.dotc.util.Stats$.track(Stats.scala:35)
	at dotty.tools.dotc.typer.Typer.typedTypeTree(Typer.scala:1056)
	at dotty.tools.dotc.typer.Typer.typedUnnamed$1(Typer.scala:1653)
	at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:1680)
	at dotty.tools.dotc.typer.Typer.$anonfun$typed$2(Typer.scala:1697)
	at dotty.tools.dotc.reporting.Reporting.traceIndented(Reporter.scala:140)
	at dotty.tools.dotc.reporting.Reporting.traceIndented$(Reporter.scala:139)
	at dotty.tools.dotc.core.Contexts$Context.traceIndented(Contexts.scala:57)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:1693)
	at dotty.tools.dotc.typer.Namer.$anonfun$typedAheadType$1(Namer.scala:940)
	at dotty.tools.dotc.typer.Namer.typedAheadImpl(Namer.scala:933)
	at dotty.tools.dotc.typer.Namer.typedAheadType(Namer.scala:940)
	at dotty.tools.dotc.typer.Namer.valOrDefDefSig(Namer.scala:1116)
	at dotty.tools.dotc.typer.Namer$Completer.typeSig(Namer.scala:732)

[allanrenucci]

I can confirm that we can now compile all the postfix _ in scalatest under scala2 mode.

Now this compile under scala2 mode:

class Test {
  def foo(fun: () => Unit) = ???

  def bar(fun: => Int) = foo(fun _) // accepted by scalac
  foo(1 _) // rejected by scalac
}

You said previously that it was a bug in scalac. I guess it is fine to support this cases under scala2 mode (even if it is a bug) if we reject them under "normal" mode

[allanrenucci]

I think we should also compile it under scala2 mode, since it only crashed in this mode

[odersky]

OK. Can you make the change and merge the PR when done? Thanks!

[allanrenucci]

Sure. Will do

[allanrenucci]

Superseded by #3402