[2.12] Implicits diverge on the first compilation but not in recompilation

[kubukoz]

reproduction steps

See repository and open Demo.scala for instructions.

problem

The compilation fails (both in bloop and sbt) with

diverging implicit expansion for type org.scalacheck.Arbitrary[(Long, Long)]
starting with method arbContainer2 in trait ArbitraryLowPriority

but if the file containing the problematic line is successfully compiled (e.g. by commenting and uncommenting the line again), it works.

This happens on 2.12.12 but not on 2.13.0, 2.13.3 or 2.13.4 - I thought it could've been resolved by one of the issues making scala/scala-dev#405, but scala/scala#7203 (backporting them to 2.12) didn't seem to alleviate the issue.

External libraries are involved in the reproduction (both use macros, as one is shapeless and the other one interacts with magnolia) - I wasn't able to reproduce the issue without them.

Output of `-Xlog-implicits` on a clean compile

[E] [E20] src/main/scala/demo/Demo.scala:13:22
[E]       not enough arguments for method arbitrary: (implicit a: org.scalacheck.Arbitrary[(Long, Long)])org.scalacheck.Gen[(Long, Long)].
[E]       Unspecified value parameter a.
[E]       L13:   Arbitrary.arbitrary[(Long, Long)]
[E]              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
[E] [E19] src/main/scala/demo/Demo.scala:13:22
[E]       diverging implicit expansion for type org.scalacheck.Arbitrary[(Long, Long)]
[E]       starting with method arbContainer2 in trait ArbitraryLowPriority
[E]       L13:   Arbitrary.arbitrary[(Long, Long)]
[E]                                 ^
 [E18] src/main/scala/demo/Demo.scala:13:22
       scalacheck.this.Arbitrary.arbContainer is not a valid implicit value for org.scalacheck.Arbitrary[(Long, Long)] because:
       diverging implicit expansion for type org.scalacheck.Arbitrary[(Long, Long)]
       starting with method arbContainer2 in trait ArbitraryLowPriority
       L13:   Arbitrary.arbitrary[(Long, Long)]
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 [E17] src/main/scala/demo/Demo.scala:13:22
       scalacheck.this.Arbitrary.arbEnum is not a valid implicit value for org.scalacheck.Arbitrary[(Long, Long)] because:
       diverging implicit expansion for type org.scalacheck.Arbitrary[(Long, Long)]
       starting with method arbContainer2 in trait ArbitraryLowPriority
       L13:   Arbitrary.arbitrary[(Long, Long)]
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 [E16] src/main/scala/demo/Demo.scala:13:22
       scalacheck.this.Arbitrary.arbTuple2 is not a valid implicit value for org.scalacheck.Arbitrary[(Long, Long)] because:
       diverging implicit expansion for type org.scalacheck.Arbitrary[(Long, Long)]
       starting with method arbContainer2 in trait ArbitraryLowPriority
       L13:   Arbitrary.arbitrary[(Long, Long)]
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 [E15] src/main/scala/demo/Demo.scala:13:22
       scalacheck.this.Arbitrary.arbContainer2 is not a valid implicit value for org.scalacheck.Arbitrary[(Long, Long)] because:
       hasMatchingSymbol reported error: diverging implicit expansion for type org.scalacheck.Arbitrary[(Long, Long)]
       starting with method arbitrarySingleton in object Arbitraries
       L13:   Arbitrary.arbitrary[(Long, Long)]
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 [E14] src/main/scala/demo/Demo.scala:13:22
       scalacheck.this.Arbitrary.arbContainer is not a valid implicit value for org.scalacheck.Arbitrary[(Long, Long)] because:
       diverging implicit expansion for type org.scalacheck.Arbitrary[(Long, Long)]
       starting with method arbitrarySingleton in object Arbitraries
       L13:   Arbitrary.arbitrary[(Long, Long)]
                                 ^
 [E13] src/main/scala/demo/Demo.scala:13:22
       scalacheck.this.Arbitrary.arbEnum is not a valid implicit value for org.scalacheck.Arbitrary[(Long, Long)] because:
       diverging implicit expansion for type org.scalacheck.Arbitrary[(Long, Long)]
       starting with method arbitrarySingleton in object Arbitraries
       L13:   Arbitrary.arbitrary[(Long, Long)]
                                 ^
 [E12] src/main/scala/demo/Demo.scala:13:22
       scalacheck.this.Arbitrary.arbTuple2 is not a valid implicit value for org.scalacheck.Arbitrary[(Long, Long)] because:
       diverging implicit expansion for type org.scalacheck.Arbitrary[(Long, Long)]
       starting with method arbitrarySingleton in object Arbitraries
       L13:   Arbitrary.arbitrary[(Long, Long)]
                                 ^
 [E11] src/main/scala/demo/Demo.scala:13:22
       scalacheck.this.Arbitrary.arbContainer2 is not a valid implicit value for org.scalacheck.Arbitrary[(Long, Long)] because:
       hasMatchingSymbol reported error: diverging implicit expansion for type org.scalacheck.Arbitrary[(Long, Long)]
       starting with method arbContainer2 in trait ArbitraryLowPriority
       L13:   Arbitrary.arbitrary[(Long, Long)]
                                 ^
 [E10] src/main/scala/demo/Demo.scala:13:22
       arbitrarySingleton is not a valid implicit value for org.scalacheck.Arbitrary[(Long, Long)] because:
       hasMatchingSymbol reported error: could not find implicit value for evidence parameter of type shapeless.Witness.Aux[(Long, Long)]
       L13:   Arbitrary.arbitrary[(Long, Long)]
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 [E9] src/main/scala/demo/Demo.scala:13:22
      shapeless.this.Witness.apply is not a valid implicit value for shapeless.Witness.Aux[(Long, Long)] because:
      hasMatchingSymbol reported error: Type argument (Long, Long) is not a singleton type
      L13:   Arbitrary.arbitrary[(Long, Long)]
                                ^
 [E8] src/main/scala/demo/Arbitraries.scala:17:19
      scalacheck.this.Arbitrary.arbEnum is not a valid implicit value for org.scalacheck.Arbitrary[Map[K,V]] because:
      typing TypeApply reported errors for the implicit tree: type arguments [Map[K,V]] do not conform to method arbEnum's type parameter bounds [A <: Enum[A]]
      L17:       Gen.resultOf(??? : T)
                 ^^^^^^^^^^^^^^^^^^^^^
 [E7] src/main/scala/demo/Arbitraries.scala:17:19
      materializing requested scala.reflect.type.ClassTag[Map[K,V]] using scala.reflect.`package`.materializeClassTag[Map[K,V]]()
      L17:       Gen.resultOf(??? : T)
                             ^
 [E6] src/main/scala/demo/Arbitraries.scala:17:19
      arbitrarySingleton is not a valid implicit value for org.scalacheck.Arbitrary[Map[K,V]] because:
      hasMatchingSymbol reported error: could not find implicit value for evidence parameter of type shapeless.Witness.Aux[Map[K,V]]
      L17:       Gen.resultOf(??? : T)
                 ^^^^^^^^^^^^^^^^^^^^^
 [E5] src/main/scala/demo/Arbitraries.scala:17:19
      shapeless.this.Witness.apply is not a valid implicit value for shapeless.Witness.Aux[Map[K,V]] because:
      hasMatchingSymbol reported error: Type argument scala.collection.immutable.Map[K,V] is not a singleton type
      L17:       Gen.resultOf(??? : T)
                             ^
 [E4] src/main/scala/demo/Arbitraries.scala:17:19
      apply is not a valid implicit value for org.scalacheck.Arbitrary[Map[K,V]] because:
      hasMatchingSymbol reported error: magnolia: could not infer ArbitraryDerivation.Typeclass for type Map[K,V]
      L17:       Gen.resultOf(??? : T)
                 ^^^^^^^^^^^^^^^^^^^^^
 [E3] src/main/scala/demo/Arbitraries.scala:17:19
      materializing requested scala.reflect.type.ClassTag[Any] using scala.reflect.`package`.materializeClassTag[Any]()
      L17:       Gen.resultOf(??? : T)
                             ^
 [E2] src/main/scala/demo/Arbitraries.scala:17:19
      arbitrarySingleton is not a valid implicit value for org.scalacheck.Arbitrary[(K, V)] because:
      hasMatchingSymbol reported error: could not find implicit value for evidence parameter of type shapeless.Witness.Aux[(K, V)]
      L17:       Gen.resultOf(??? : T)
                 ^^^^^^^^^^^^^^^^^^^^^
 [E1] src/main/scala/demo/Arbitraries.scala:17:19
      shapeless.this.Witness.apply is not a valid implicit value for shapeless.Witness.Aux[(K, V)] because:
      hasMatchingSymbol reported error: Type argument (K, V) is not a singleton type
      L17:       Gen.resultOf(??? : T)
                             ^
[E] src/main/scala/demo/Demo.scala: L13 [E9], L13 [E10], L13 [E11], L13 [E12], L13 [E13], L13 [E14], L13 [E15], L13 [E16], L13 [E17], L13 [E18], L13 [E19], L13 [E20]
[E] src/main/scala/demo/Arbitraries.scala: L17 [E1], L17 [E2], L17 [E3], L17 [E4], L17 [E5], L17 [E6], L17 [E7], L17 [E8]

Output of -Xlog-implicits when the line is being added back

 [E8] src/main/scala/demo/Demo.scala:13:22
      scalacheck.this.Arbitrary.arbEnum is not a valid implicit value for org.scalacheck.Arbitrary[(Long, Long)] because:
      typing TypeApply reported errors for the implicit tree: type arguments [(Long, Long)] do not conform to method arbEnum's type parameter bounds [A <: Enum[A]]
      L13:   Arbitrary.arbitrary[(Long, Long)]
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 [E7] src/main/scala/demo/Demo.scala:13:22
      materializing requested scala.reflect.type.ClassTag[(Long, Long)] using scala.reflect.`package`.materializeClassTag[(Long, Long)]()
      L13:   Arbitrary.arbitrary[(Long, Long)]
                                ^
 [E6] src/main/scala/demo/Demo.scala:13:22
      scalacheck.this.Arbitrary.arbEnum is not a valid implicit value for org.scalacheck.Arbitrary[Long] because:
      typing TypeApply reported errors for the implicit tree: type arguments [Long] do not conform to method arbEnum's type parameter bounds [A <: Enum[A]]
      L13:   Arbitrary.arbitrary[(Long, Long)]
                                ^
 [E5] src/main/scala/demo/Demo.scala:13:22
      materializing requested scala.reflect.type.ClassTag[Long] using scala.reflect.`package`.materializeClassTag[Long]()
      L13:   Arbitrary.arbitrary[(Long, Long)]
                                ^
 [E4] src/main/scala/demo/Demo.scala:13:22
      arbitrarySingleton is not a valid implicit value for org.scalacheck.Arbitrary[Long] because:
      hasMatchingSymbol reported error: could not find implicit value for evidence parameter of type shapeless.Witness.Aux[Long]
      L13:   Arbitrary.arbitrary[(Long, Long)]
                                ^
 [E3] src/main/scala/demo/Demo.scala:13:22
      shapeless.this.Witness.apply is not a valid implicit value for shapeless.Witness.Aux[Long] because:
      hasMatchingSymbol reported error: Type argument Long is not a singleton type
      L13:   Arbitrary.arbitrary[(Long, Long)]
                                ^
 [E2] src/main/scala/demo/Demo.scala:13:22
      arbitrarySingleton is not a valid implicit value for org.scalacheck.Arbitrary[(Long, Long)] because:
      hasMatchingSymbol reported error: could not find implicit value for evidence parameter of type shapeless.Witness.Aux[(Long, Long)]
      L13:   Arbitrary.arbitrary[(Long, Long)]
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 [E1] src/main/scala/demo/Demo.scala:13:22
      shapeless.this.Witness.apply is not a valid implicit value for shapeless.Witness.Aux[(Long, Long)] because:
      hasMatchingSymbol reported error: Type argument (Long, Long) is not a singleton type
      L13:   Arbitrary.arbitrary[(Long, Long)]
                                ^
src/main/scala/demo/Demo.scala: L13 [E1], L13 [E2], L13 [E3], L13 [E4], L13 [E5], L13 [E6], L13 [E7], L13 [E8]

[kubukoz]

FYI still happening in 2.12.17, but not in 2.13.10

[SethTisue]

We don't keep open tickets on 2.12-only bugs (this one slipped through the cracks).

Does that mean we wouldn't accept a 2.12-only PR with a fix? Well, we might... it would really depend on a risk/reward assessment depending on the nature of the fix.