Overloading or implicit bug in 3.4.1

[soronpo]

This regression in 3.4.1 first caused me to chase down the wrong rabbit hole of #19955, so it could be related somehow, but I doubt it.
This is a very weird bug, that can disappear if we remove one of the following:

• Remove the def ^^^ : Unit = ???
• Remove the (using check: c.OutW =:= c.OutW) from the third def ^^^ declaration
  The third ^^^ definition should not be considered for overloading at all because the first one is the more specific.

The error also disappear if we split x ^^^ x ^^^ x into two separate vals.

Compiler version

Last good release: 3.4.1-RC1-bin-20240126-1716bcd-NIGHTLY
First bad release: 3.4.1-RC1-bin-20240129-b20747d-NIGHTLY
bisect: cce2933 in #19096

Minimized code

trait Summon[R, T <: R]:
  type Out
object Summon:
  given [R, T <: R]: Summon[R, T] with
    type Out = R

sealed class Modifier[+A, +P]
type ModifierAny = Modifier[Any, Any]
sealed trait ISCONST[T <: Boolean]
type CONST = ISCONST[true]

trait DFTypeAny
trait DFBits[W <: Int] extends DFTypeAny
trait DFVal[+T <: DFTypeAny, +M <: ModifierAny]
type DFValAny = DFVal[DFTypeAny, ModifierAny]
type DFValTP[+T <: DFTypeAny, +P] = DFVal[T, Modifier[Any, P]]
type DFConstOf[+T <: DFTypeAny] = DFVal[T, Modifier[Any, CONST]]

trait Candidate[R]:
  type OutW <: Int
  type OutP
object Candidate:
  given [W <: Int, P, R <: DFValTP[DFBits[W], P]]: Candidate[R] with
    type OutW = W
    type OutP = P

extension [L <: DFValAny](lhs: L)(using icL: Candidate[L])
  def ^^^[R](rhs: R)(using
      icR: Candidate[R]
  ): DFValTP[DFBits[icL.OutW], icL.OutP | icR.OutP] = ???
  def ^^^ : Unit = ???
extension [L](lhs: L)
  def ^^^[RW <: Int, RP](
      rhs: DFValTP[DFBits[RW], RP]
  )(using es: Summon[L, lhs.type])(using
      c: Candidate[L]
  )(using check: c.OutW =:= c.OutW): DFValTP[DFBits[c.OutW], c.OutP | RP] = ???

val x: DFConstOf[DFBits[8]] = ???
val zzz = x ^^^ x ^^^ x

Output

   |val zzz = x ^^^ x ^^^ x
   |                       ^
   |No given instance of type Summon[DFValTP[DFBits[?1.OutW], ?2.OutP | ISCONST[(true : Boolean)]],
   |  (?3 : DFValTP[DFBits[?1.OutW], ?2.OutP | ISCONST[(true : Boolean)]])] was found for parameter es of method ^^^.
   |I found:
   |
   |    Summon.given_Summon_R_T[R, T]
   |
   |But given instance given_Summon_R_T in object Summon does not match type Summon[DFValTP[DFBits[?1.OutW], ?2.OutP | ISCONST[(true : Boolean)]],
   |  (?3 : DFValTP[DFBits[?1.OutW], ?2.OutP | ISCONST[(true : Boolean)]])]
   |
   |where:    ?1 is an unknown value of type Candidate.given_Candidate_R[(8 : Int), ISCONST[(true : Boolean)],
   |  DFVal[DFBits[(8 : Int)], Modifier[Any, CONST]]]
   |          ?2 is an unknown value of type Candidate.given_Candidate_R[(8 : Int), ISCONST[(true : Boolean)],
   |  DFVal[DFBits[(8 : Int)], Modifier[Any, CONST]]]
   |          ?3 is an unknown value of type DFValTP[DFBits[?1.OutW], ?2.OutP | ISCONST[(true : Boolean)]]

Expectation

No error.

[EugeneFlesselle]

Minimised a bit more:

trait DFVal[+T <: Int, +P]

trait Summon[R, T <: R]
given [R, T <: R]: Summon[R, T] with {}

trait Candidate[R]:
  type OutW <: Int
  type OutP
given [W <: Int, P, R <: DFVal[W, P]]: Candidate[R] with
  type OutW = W
  type OutP = P

extension [L <: DFVal[Int, Any]](lhs: L)(using icL: Candidate[L])
  def ^^^[R](rhs: R)
            (using icR: Candidate[R])
            : DFVal[icL.OutW, icL.OutP | icR.OutP] = ???
  def ^^^ : Unit = ???

extension [L](lhs: L)
  def ^^^[RW <: Int, RP](rhs: DFVal[RW, RP])
                        (using es: Summon[L, lhs.type])
                        (using c: Candidate[L])
                        (using check: c.OutW =:= c.OutW)
                        : DFVal[c.OutW, c.OutP | RP] = ???

val x: DFVal[8, true] = ???
val z1 = x ^^^ x // Ok
val z2 = z1 ^^^ x // Ok
val zzz = x ^^^ x ^^^ x // Error

When def ^^^ : Unit = ??? is removed, all of z1, z2, zzz succeed with the first def ^^^.
When it is present, they all attempt to use last def ^^^ which fails for zzz only.
If we remove the last def ^^^, then they all succeed with the first one independently of the 2nd one.

[soronpo]

I will say that further look suggests there is a (false?) overloading ambiguity issue that may cause this:

trait DFVal[+T <: Int, +P]
type DFValAny = DFVal[Int, Any]
trait Candidate[R]:
  type OutW <: Int
  type OutP
given [W <: Int, P, R <: DFVal[W, P]]: Candidate[R] with
  type OutW = W
  type OutP = P

val x = new DFVal[5, true]{}

object Works1:
  extension [L <: DFValAny](lhs: L)(using icL: Candidate[L])
    def ^^^(rhs: Any): Unit = ???
  extension [L](lhs: L)
    def ^^^[R](rhs: R): Unit = ???
  val y = x ^^^ x //ok

object Works2:
  extension [L <: DFValAny](lhs: L)(using icL: Candidate[L])
    def ^^^(rhs: Any): Unit = ???
    def ^^^ : Unit = ???
  val y = x ^^^ x //ok

object Fail:
  extension [L <: DFValAny](lhs: L)(using icL: Candidate[L])
    def ^^^(rhs: Any): Unit = ???
    def ^^^ : Unit = ???
  extension [L](lhs: L)
    def ^^^[R](rhs: R): Unit = ???
  val y = x ^^^ x //error

[soronpo]

OK, so if I understand correctly, the reason my original code works now on nightly is that #19955 was resolved. But there is still an underlying overloading issue, right?

[EugeneFlesselle]

OK, so if I understand correctly, the reason my original code works now on nightly is that #19955 was resolved. But there is still an underlying overloading issue, right?

Yes that's right, it compiles but picks the wrong (3rd) alternative.

[sjrd]

Based on my analysis at #20054 (comment), it appears adjusting overload resolution to be intuitive wrt. to this issue would definitely be a breaking change.

@soronpo You mention that

my original code works now on nightly

so the current issue is not a regression per se after all, is it?

[soronpo]

so the current issue is not a regression per se after all, is it?

You are correct. The part that has regressed was fixed. I do think the overloading needs to be fixed, but if we can't tie the knot of the rules well, then we should avoid further regressions first.

[Gedochao]

This has been decided to be worked on in 3.7.0 (TBD what's the timing on that).
For now, we will add a warning in 3.6.0.

[soronpo]

What kind of warning? It's already an error. I would caution from adding redundant warnings. They only increase technical debt with false positives and specific version suppression. Better no action until 3.7.

[EugeneFlesselle]

It's already an error.

It is not in cases such as those discussed in #20054 (comment) and #20054 (comment)

What kind of warning?

We plan of warning (in 3.6.0) about the changes in overload resolution which would apply in 3.7.0.

[soronpo]

@Gedochao was warning applied in 3.6.x? I don't think I saw a PR fo it.