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implicit resolution error for Aux pattern with parametrized type #10528

scalasolist opened this Issue Sep 28, 2017 · 1 comment


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3 participants

scalasolist commented Sep 28, 2017


The error shows itself when implicit resolution tried to find Aux pattern for a type that has parameter. The same case without Aux pattern or without type parameter compiles fine. I expect that more complicated example should also compile, but it fails.


Error reproduction

The error could be reproduced with the following code:

trait Holder[A]
trait NilHolder[A] extends Holder[A]

trait Solve[A, H <: Holder[A]] {
  type Output <: Holder[A]
type SolveAux[A, H <: Holder[A], O <: Holder[A]] = Solve[A, H] {type Output = O}

implicit def nilSolve[A] = new Solve[A, NilHolder[A]] {
  override type Output = NilHolder[A]

trait WrapSolve[A, H <: Holder[A]] {
  type Output <: Holder[A]

implicit def wrapAux[A, H <: Holder[A], O <: Holder[A]](implicit one : SolveAux[A, H, O]) =
  new WrapSolve[A, H] {
    override type Output = O

val wrapped = implicitly[WrapSolve[String, NilHolder[String]]]

Error message

OutsideImplicitTree.scala:30: error: could not find implicit value for parameter e: Parametrized.WrapSolve[String,Parametrized.NilHolder[String]]
  val wrapped = implicitly[WrapSolve[String, NilHolder[String]]]
one error found

I tried to -Xlog-implicits and it showed quite useful info. nilSolve was successfully adopted as implicit argument, but typechecker suddenly failed to recognized its type:

OutsideImplicitTree.scala:30: nilSolve is not a valid implicit value for Parametrized.SolveAux[String,Parametrized.NilHolder[String],O] because:
type parameters weren't correctly instantiated outside of the implicit tree: inferred type arguments [this.Output] do not conform to method wrapAux's type parameter bounds [O <: Parametrized.Holder[A]]
  val wrapped = implicitly[WrapSolve[String, NilHolder[String]]]


If you chose to abandon Aux pattern compiler recognizes types immediately:

implicit def wrapNoAux[A, H <: Holder[A]](implicit one : Solve[A, H]) =
  new WrapSolve[A, H] {
    override type Output = one.Output

When workaround does not work

This workaround works but is not universally applicable. For example, what If I need to perform some transformation twice?

trait TwoTimes[A, H <: Holder[A]] {
  type Output <: Holder[A]

implicit def twoTimes[A, H <: Holder[A], O <: Holder[A]](implicit
  one : SolveAux[A, H, O],
  two : Solve[A, O]
) = new TwoTimes[A, H] {
  override type Output = two.Output

Comparing to simple type

If the Holder type is not parametrized the entire thing works flawlessly:

trait Holder
trait NilHolder extends Holder

trait Solve[A, H <: Holder] {
  type Output <: Holder
type SolveAux[A, H <: Holder, O <: Holder] = Solve[A, H] {type Output = O}

implicit def nilSolve[A] = new Solve[A, NilHolder] {
  override type Output = NilHolder

trait WrapSolve[A, H <: Holder] {
  type Output <: Holder

implicit def wrapAux[A, H <: Holder, O <: Holder](implicit one : SolveAux[A, H, O]) =
  new WrapSolve[A, H] {
    override type Output = O

val wrapped = implicitly[WrapSolve[String, NilHolder]]

This comment has been minimized.

milessabin commented Oct 19, 2017

This is a simplified manifestation of the same underlying issue,

object Test2 {
  class Inv[T]
  class Foo[T, U <: Inv[T]]

  implicit def foo[T]: Foo[T, Inv[T]] = new Foo[T, Inv[T]]

  def bar[T, U <: Inv[T]](implicit foo: Foo[T, U]): Inv[T] = new Inv[T]

  val baz: Inv[Int] = bar

The problem is that when T is solved as Int when bar is applied that isn't propagated to the bound of U.

@milessabin milessabin self-assigned this Oct 19, 2017

milessabin added a commit to milessabin/scala that referenced this issue Jan 12, 2018

Use solved type vars to improve bounds of undetermined vars
When instantiating type parameters and retracting unsolved ones via
adjustTypeArgs and friends we end up with a list of solved type
params/variables and a list of still-undetermined type parameters.
Typically this is followed by a substitution of the solved variables
into the positions of the corresponding parameters, and type checking
proceeds with the substitutions eliminated from the list of undetermined
type parameters.

However, the substituted parameters might occur as components of the
bounds of the type parameters which are yet to be determined and, prior
to this commit, these occurrences are not substituted into with the solved
variables. This leads to issues of the form seen in scala/bug#10528.
This commit performs the substitution similarly to the way it is done in
enhanceBounds in typedAppliedTypeTree and fixes scala/bug#10528.

The new addition is largely a duplicate of the existing mechanism in
part to enable it to use updateInfo rather than setInfo and so preserve
type histories. There is a TODO to investigate whether the original
mechanism should also attempt to preserve type histories and whether the
two methods can be combined.

@SethTisue SethTisue added this to the 2.13.0-M3 milestone Jan 16, 2018

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