Allow using classOf with object type

[octonato]

This is a fix for scala/bug#2453.

It makes it possible to write the following:

object Bar 

val cls = classOf[Bar.type]

[SethTisue]

(iirc there is a passage in the spec that will need to be updated)

[SethTisue]

@odersky you aren't opposed to this on principle, by any chance?

[octonato]

Left I few comments to motivate the change.

[octonato]

At the end, this seems to be suspiciously simple fix. Not sure if that has other implications though.

[octonato]

I added this compile test because the issue shows up when using a Java annotation that requires a class. But of course, the impl allows many other usages.

When I faced that issue for the first time, I tried to used it as follows:

trait Foo {
  @AnnotationWithClassType(cls = Bar.getClass)
  def function: Any = ???
}

which gives the following error:

Foo.scala:5: error: annotation argument needs to be a constant; found: Bar.getClass()
  @AnnotationWithClassType(cls = Bar.getClass)

[som-snytt]

How about classOf[23], who are second-year students now?

I just worked around this:

scala> object X
object X

scala> classOf[X.type]
                ^
       error: class type required but X.type found

scala> def f[A: reflect.ClassTag] = implicitly[reflect.ClassTag[A]].runtimeClass
def f[A](implicit evidence$1: scala.reflect.ClassTag[A]): Class[_]

scala> f[X.type]
val res1: Class[_] = class X$

Edit: I see the macro version in the linked ticket.

[dwijnand]

I just worked around this:

Yeah, that's user-land classOf (https://github.com/dwijnand/psp-std/blob/d6c01d6067f241d009dc4509ef85b32958aff8f6/std/src/main/scala/std/Api.scala#L67)

[octonato]

def f[A: reflect.ClassTag] = implicitly[reflect.ClassTag[A]].runtimeClass

This won't work with Java annotation for the same reason that @AnnotationWithClassType(cls = Bar.getClass) doesn't work.

object Bar

object Companion {
  def classOf[A: reflect.ClassTag] = implicitly[reflect.ClassTag[A]].runtimeClass
}

trait Foo {
  @AnnotationWithClassType(cls = Companion.classOf[Bar.type])
  def function: Any = ???
}

gives:

Foo.scala:9: error: annotation argument needs to be a constant; found: Companion.classOf[Bar.type]((ClassTag.apply[Bar.type](classOf[Bar$]): scala.reflect.ClassTag[Bar.type]))
  @AnnotationWithClassType(cls = Companion.classOf[Bar.type])

[dwijnand]

So short and sweet!

Making isNonRefinementClassType return true for SingleType's of module symbols doesn't sound like a problem to me. Hopefully it's good for the test suite too. Needs a rebase on #9046 #9280 (or 2.13.x once that's merged).

[octonato]

I don't get why it needs a rebase on #9046. Did you pasted here the right issue number?

Btw, the tests are failing for an unrelated compilation issue.

[error] /home/travis/build/scala/scala/src/compiler/scala/tools/nsc/transform/Constructors.scala:477:25: Unused import
[error]       import scala.util.Try
[error]                         ^
[error] one error found
[error] (compiler / Compile / compileIncremental) Compilation failed

Is 2.13.x branch broken? Or maybe your intention was to link here the PR that fix 2.13 branch?

[octonato]

I guess this was the PR you wanted to link: #9280

[octonato]

@dwijnand, this is rebased with #9280.
Let's what TravisCI will tell us.

[octonato]

TravisCI failed with some other unrelated reason. I will try again in a few days (after rebasing on 2.13).

Also, before adding more time on this, I would like to know if the solution is sound.

[dwijnand]

Also, before adding more time on this, I would like to know if the solution is sound.

It looks good to me, so it has a reasonable chance to be merged.

[SethTisue]

codewise LGTM but I need to dig deeper into whether a spec change is needed. will do soon nah, spec is fine

@smarter any objection to making the corresponding change to Scala 3?

[smarter]

It already works in dotty :).

[SethTisue]

thank you Renato!

you get a special prize for closing such a low bug number! (*)

(* there is no actual prize)