Expr.summon does not work with imported givens from a class instance

[deusaquilus]

Compiler version

3.0.0-RC1
also on 3.0.0-RC2-bin-20210227-07d9dd2-NIGHTLY

Minimized code

Create some kind of typeclass with an implementation inside of a class-instance

trait MyEncoder[T] {
  def encode(t: T): String
}

class MyContext {
  given intEncoder: MyEncoder[Int] = new MyEncoder[Int] { def encode(t: Int) = s"${t}" }
}

Then make a macro that summons this using Expr.summon.

object MyMacro {
  inline def getAndEncode[T](t: T): String = ${ getAndEncodeImpl('t) }
  def getAndEncodeImpl[T: Type](t: Expr[T])(using qctx: Quotes): Expr[String] = {
    import qctx.reflect._
    val enc = 
      Expr.summon[MyEncoder[T]] match
        case Some(enc) => enc
        case None => report.throwError("Can't find encoder")
    '{ $enc.encode($t) }
  }
}

Then import the context and use the macro:

  def doEncoding: Unit = {
    val ctx = new MyContext()
    import ctx.{*, given}
    println(MyMacro.getAndEncode(123))
  }

Output

Compilation fails, report error case "Can't find encoder" is hit.

[error] 15 |    println(MyMacro.getAndEncode(123))
[error]    |            ^^^^^^^^^^^^^^^^^^^^^^^^^
[error]    |            Can't find encoder

Expectation

Compile should work and the given should be correctly summoned.

Note that if instead of using a macro I use a regular method, it will work:

  def encodeIt[T](t: T)(using enc: MyEncoder[T]) = enc.encode(t)

  def doEncoding: Unit = {
    val ctx = new MyContext()
    import ctx.{*, given}
    println(encodeIt(123))
  }

Why the discrepancy?

Also, note that if you declare the given directly in the doEncoding method it will work:

  def doEncoding: Unit = {
    val ctx = new MyContext()
    given intEncoder: MyEncoder[Int] = new MyEncoder[Int] { def encode(t: Int) = s"${t}" }
    println(MyMacro.getAndEncode(123))
  }

Why the discrepancy?

Code

Full code sample is available here:
https://github.com/deusaquilus/expr_summon_issue

[deusaquilus]

If I'm not totally mistaken and this is a real issue, RC1 is totally unusable for me 😢

[deusaquilus]

Okay, so this doesn't work with object either. Say that I do this:

object Ctx {
  given intEncoder: MyEncoder[Int] = new MyEncoder[Int] { def encode(t: Int) = s"${t}" }
}

Then I do this:

  def doEncoding: Unit = {
    import Ctx.{given,*}
    println(MyMacro.getAndEncode(123))
  }

It does not work either:

[error] 20 |    println(MyMacro.getAndEncode(123))
[error]    |            ^^^^^^^^^^^^^^^^^^^^^^^^^
[error]    |            Can't find encoder

[deusaquilus]

@nicolasstucki @liufengyun @odersky
This really, really scares me!
I was building just fine on the Feb 02 build (20210202-8d43a9c-NIGHTLY), then comes the RC1 release and Boom! This bug happens and everything I'm doing is completely broken.
I keep thinking, what if something like this happens in the 3.0.0 release!?

For the 3.0.0 release, can we have some kind of Scala 3 stage-gate making sure Dotty-Quill builds before the release is cut? Dotty-Quill seems to be the only thing constantly blowing up on the metaprogramming bugs in new releases. So it really needs to be double-checked.
Or maybe we bring Dotty-Quill into the Community Build ASAP and do it that way?

[nafg]

I think usually a release is an actual release candidate that has no issues reported for a few weeks

…

On Sun, Feb 28, 2021, 1:24 PM Alexander Ioffe ***@***.***> wrote: @nicolasstucki <https://github.com/nicolasstucki> @liufengyun <https://github.com/liufengyun> @odersky <https://github.com/odersky> This really, really scares me! I was building just fine on the Feb 02 build (20210202-8d43a9c-NIGHTLY), then comes the RC1 release and Boom! This bug happens and everything I'm doing is completely broken. I keep thinking, what if something like this happens in the 3.0.0 release!? For the 3.0.0 release, can we have some kind of Scala 3 stage-gate making sure Dotty-Quill builds before the release is cut? Dotty-Quill seems to be the only thing constantly blowing up on the metaprogramming bugs in new releases. So it really needs to be double-checked. Or maybe we bring Dotty-Quill into the Community Build ASAP and do it that way? — You are receiving this because you are subscribed to this thread. Reply to this email directly, view it on GitHub <#11557 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AAAYAUCIQMGZUC3YKCDBIG3TBKC5PANCNFSM4YK7AQPA> .

[nicolasstucki]

Minimized

type MyEncoder

class MyContext:
  given intEncoder: MyEncoder = ???

def doEncoding(ctx: MyContext): Unit =
  import ctx.{*, given}
  summon[MyEncoder]
  summonInlineMyEncoder()

inline def summonInlineMyEncoder(): Unit =
  compiletime.summonInline[MyEncoder]

[SethTisue]

maybe we bring Dotty-Quill into the Community Build ASAP and do it that way

that certainly sounds like the right thing to do

it's also possible to add latest-Scala-3-nightly to your own CI matrix