"Ambiguous implicit values" error caused by a mix of `-Xsource:3`, an explicit (not wildcard) import and a evicted/transitive(?) dependency

[hmemcpy]

We've hit a strange issue today, and it took me a while to track down, but I was able to reproduce it. Here's an example of the issue on Scastie: https://scastie.scala-lang.org/hmemcpy/PLSmFypOQ0arLZmTb50dfQ/7

It boils down to the following combination: compiling with -Xsource:3, a non-wildcard import, and referencing a dependency directly and indirectly via another one. I was able to reproduce this with the ZIO library. Given the conditions mentioned, the following code snippet doesn't compile:

import zio.ZIO

val nextRand = zio.random.nextDoubleBetween(0, 1)
ZIO.collectAll(List.fill(3)(nextRand))

failing with the following error:

ambiguous implicit values:
 both method buildFromAny in trait BuildFromCompat of type [Element, Collection[+Element] <: Iterable[Element] with scala.collection.IterableOps[Any,Collection,Any]]zio.package.BuildFrom[Collection[Any],Element,Collection[Element]]
 and method buildFromIterableOps in trait BuildFromLowPriority2 of type [CC[X] <: Iterable[X] with scala.collection.IterableOps[X, CC, _], A0, A]scala.collection.BuildFrom[CC[A0],A,CC[A]]
 match expected type zio.BuildFrom[List[zio.ZIO[zio.random.Random,Nothing,Double]],Double,List[Double]] [56:23]

reproduction steps

using Scala 2.13.4 (and 2.13.5), and the following sbt definition:

lazy val zioVersion = "1.0.4-2"
lazy val zioConfigVersion = "1.0.0"

ThisBuild / scalaVersion := "2.13.4"

lazy val root = project
  .in(file("."))
  .settings(
    libraryDependencies ++= Seq(
      "dev.zio" %% "zio" % zioVersion,
      "dev.zio" %% "zio-config" % zioConfigVersion,
    ),
    scalacOptions ++= Seq(
       "-Xsource:3"
    ),
  )

Adding a file with the code snippet above causes it to fail with "ambiguous implicit values" error. Enabling -Ydebug revealed additional errors:

[ suppressed ] Cannot construct a collection of type immutable.this.List[scala.this.Double] with elements of type scala.this.Double based on a collection of type immutable.this.List[zio.this.ZIO[<root>.this.zio.random.Random,scala.this.Nothing,scala.this.Double]].
  ZIO.collectAll(List.fill(3)(nextRand))

[ suppressed ] not enough arguments for method collectAll: (implicit bf: zio.this.BuildFrom[immutable.this.List[zio.this.ZIO[<root>.this.zio.random.Random,scala.this.Nothing,scala.this.Double]],scala.this.Double,immutable.this.List[scala.this.Double]]): zio.this.ZIO[<root>.this.zio.random.Random,scala.this.Nothing,immutable.this.List[scala.this.Double]].
Unspecified value parameter bf.
  ZIO.collectAll(List.fill(3)(nextRand))

problem analysis

The ZIO.collectAll function requires an implicit BuildFrom, and provides a conversion in the zio.BuildFromCompat trait.

The dependency zio-config depends itself on ZIO v1.4.0-2. However, when both are referenced - it causes the compilation error (even though I'm not using anything from zio-config directly). Removing zio-config from libraryDependencies does not cause the issue (I initially thought this had to do with a mismatched ZIO version being evicted, but was able to confirm this not working even with "matching" versions).

workaround

When either removing -Xsource:3 OR removing zio-config OR importing zio._ instead of just zio.ZIO - the code compiles without problems!

I'm not sure who's the culprit here...

[Jasper-M]

FWIW I get the compilation errors whether I select v1.0.4-2 or v1.0.4 in scastie.

[hmemcpy]

@Jasper-M looks like you're right - seems I made a mistake and I can reproduce this even with referencing 1.4.0-2. However, it does seem that both libraries are required - when I remove zio-config (even though I'm not using it directly) - the problem does not occur!

I'll amend my issue. Thanks!

[Jasper-M]

The trigger of the issue seems to be in zio 1.4.0-2. That version gives the error, while zio 1.4.0 doesn't (if 1.4.0-2 doesn't get pulled in transitively that is).

And it looks like that buildFromAny method in the error message was added between those 2 versions.

I guess you'd have to cross-compile with actual Scala 3 to know if it's actually some kind of bug in -Xsource:3 or if it's caused by a legitimate change in implicit resolution in Scala 3.

[hmemcpy]

@Jasper-M looks like you're right again, I hadn't even looked at when the buildFromAny was added (seems January 22nd). Let me try cross-compiling and minimizing this even further. Cheers!

[Jasper-M]

I guess you'd have to cross-compile with actual Scala 3 to know if it's actually some kind of bug in -Xsource:3 or if it's caused by a legitimate change in implicit resolution in Scala 3.

Though I have to add that I doubt that all changes that affect implicit resolution have been ported to -Xsource:3.

[som-snytt]

The big/only change is variance. (No other changes around shadowing, nesting.)

It's a good interview question, whether a single import can be more ambiguous than a wildcard.

I notice the zio test is in the zio package. I have noticed elsewhere that this convention means we don't test the imports ergonomics of the user.

[nartamonov]

I've got the same issue with Scala 2.13.5 and ZIO 1.0.5 and -Xsource:3:

I haven't zio-config in classpath, but have zio-interop-cats 2.3.1.0 and zio-test 1.0.5

Wildcard import zio._ instead of zio.ZIO fixed problem.

[joroKr21]

It's also messing up Intellij highlighting

[SethTisue]

is there a standalone reproduction that doesn't involve an external dependency?

[joroKr21]

That's an issue in ZIO IMHO. It directly defines:

implicit def buildFromAny[Element, Collection[+Element] <: Iterable[Element] with IterableOps[Any, Collection, Any]]
    : BuildFrom[Collection[Any], Element, Collection[Element]] =
    scala.collection.BuildFrom.buildFromIterableOps[Collection, Any, Element]

But there is no need - BuildFrom.buildFromIterableOps is already implicit

[joroKr21]

See #12104 and zio/zio#4582

[SethTisue]

closing — but of course happy to reopen if this is found to be somehow actionable in scala/scala