Quoted patterns regression: match with Underlying type fails

[hmf]

Compiler version

Compiler version 3.2.1

Minimized code

This is the code that worked when discussed in issue #10358. I am reporting it here so that it does not slip through the cracks (discussed in November 2020):

  import scala.quoted.*

  sealed trait HList
  case class HCons[+HD, TL <: HList](hd: HD, tl: TL) extends HList
  case object HNil extends HList

  def showFirstTwoImpl(e: Expr[HList])(using Quotes): Expr[String] = {
    e match {
      // Ok. No match 
      case '{HCons($h1, HCons($h2, $_))} => '{$h1.toString ++ $h2.toString}
      // Fails. Does not compile 
      case '{HCons($h1: hd1, HCons($h2: hd2, $_ : tl))} => '{$h1.toString ++ $h2.toString}
      // Ok
      case '{HCons[hd, HCons[sd, tl]]($h1, HCons($h2, $_))} => '{$h1.toString ++ $h2.toString}
      case _ => '{""}
    }
  }

  transparent inline def showFirstTwo(inline xs: HList) = ${ showFirstTwoImpl('xs) }  

Output

error] -- [E007] Type Mismatch Error: /home/hmf/VSCodeProjects/sploty/meta/src/data/Macros3.scala:1184:45 
[error] 1184 |      case '{HCons($h1: hd1, HCons($h2: hd2, $_ : tl))} => '{$h1.toString ++ $h2.toString}
[error]      |                                             ^^^^^^^
[error]      |                                      Found:    tl$given2.Underlying
[error]      |                                      Required: data.Macros3.HList
[error]      |--------------------------------------------------------------------------
[error]      | Explanation (enabled by `-explain`)

Compiler explanation:

error]      |
[error]      | Tree: ${_}:(tl$given2 @ _).Underlying
[error]      | I tried to show that
[error]      |   tl$given2.Underlying
[error]      | conforms to
[error]      |   TL
[error]      | but the comparison trace ended with `false`:
[error]      |
[error]      |   ==> tl$given2.Underlying  <:  TL
[error]      |     ==> tl$given2.Underlying  <:  TL
[error]      |       ==> tl  <:  TL
[error]      |         ==> tl  <:  TL in frozen constraint
[error]      |           ==> tl  <:  Nothing in frozen constraint
[error]      |             ==> Any  <:  Nothing (left is approximated) in frozen constraint
[error]      |             <== Any  <:  Nothing (left is approximated) in frozen constraint = false
[error]      |           <== tl  <:  Nothing in frozen constraint = false
[error]      |           ==> Any  <:  TL (left is approximated) in frozen constraint
[error]      |             ==> Any  <:  Nothing (left is approximated) in frozen constraint
[error]      |             <== Any  <:  Nothing (left is approximated) in frozen constraint = false
[error]      |           <== Any  <:  TL (left is approximated) in frozen constraint = false
[error]      |         <== tl  <:  TL in frozen constraint = false
[error]      |         ==> add constraint TL >: tl , constraint =  uninstantiated variables: TL, HD, TL, HD, TL, HD, TL, HD, TL  constrained types:    [HD, TL <: data.Macros3.HList](hd: HD, tl: TL): data.Macros3.HCons[HD, TL] , [HD, TL <: data.Macros3.HList](hd: HD, tl: TL): data.Macros3.HCons[HD, TL],    [HD, TL <: data.Macros3.HList](hd: HD, tl: TL): data.Macros3.HCons[HD, TL] , [HD, TL <: data.Macros3.HList](hd: HD, tl: TL): data.Macros3.HCons[HD, TL],    [HD, TL <: data.Macros3.HList](hd: HD, tl: TL): data.Macros3.HCons[HD, TL]  bounds:       HD := Any      TL >: data.Macros3.HCons[Any, data.Macros3.HList] <: data.Macros3.HList      HD      TL <: data.Macros3.HList      HD >: hd1      TL >: data.Macros3.HCons[HD, TL] <: data.Macros3.HList      HD      TL <: data.Macros3.HList      HD >: hd2      TL <: data.Macros3.HList  ordering: 
[error]      |           ==> lub(Nothing, tl, canConstrain=false, isSoft=true)
[error]      |           <== lub(Nothing, tl, canConstrain=false, isSoft=true) = tl
[error]      |           ==> tl  <:  data.Macros3.HList
[error]      |             ==> Any  <:  data.Macros3.HList (left is approximated)
[error]      |             <== Any  <:  data.Macros3.HList (left is approximated) = false
[error]      |           <== tl  <:  data.Macros3.HList = false
[error]      |         <== add constraint TL >: tl , constraint =  uninstantiated variables: TL, HD, TL, HD, TL, HD, TL, HD, TL  constrained types:    [HD, TL <: data.Macros3.HList](hd: HD, tl: TL): data.Macros3.HCons[HD, TL] , [HD, TL <: data.Macros3.HList](hd: HD, tl: TL): data.Macros3.HCons[HD, TL],    [HD, TL <: data.Macros3.HList](hd: HD, tl: TL): data.Macros3.HCons[HD, TL] , [HD, TL <: data.Macros3.HList](hd: HD, tl: TL): data.Macros3.HCons[HD, TL],    [HD, TL <: data.Macros3.HList](hd: HD, tl: TL): data.Macros3.HCons[HD, TL]  bounds:       HD := Any      TL >: data.Macros3.HCons[Any, data.Macros3.HList] <: data.Macros3.HList      HD      TL <: data.Macros3.HList      HD >: hd1      TL >: data.Macros3.HCons[HD, TL] <: data.Macros3.HList      HD      TL <: data.Macros3.HList      HD >: hd2      TL <: data.Macros3.HList  ordering:  = false
[error]      |       <== tl  <:  TL = false
[error]      |     <== tl$given2.Underlying  <:  TL = false
[error]      |   <== tl$given2.Underlying  <:  TL = false
[error]      |
[error]      | The tests were made under a constraint with:
[error]      |  uninstantiated variables: TL, HD, TL, HD, TL, HD, TL, HD, TL
[error]      |  constrained types: 
[error]      |   [HD, TL <: data.Macros3.HList](hd: HD, tl: TL): data.Macros3.HCons[HD, TL]
[error]      | , [HD, TL <: data.Macros3.HList](hd: HD, tl: TL): data.Macros3.HCons[HD, TL], 
[error]      |   [HD, TL <: data.Macros3.HList](hd: HD, tl: TL): data.Macros3.HCons[HD, TL]
[error]      | , [HD, TL <: data.Macros3.HList](hd: HD, tl: TL): data.Macros3.HCons[HD, TL], 
[error]      |   [HD, TL <: data.Macros3.HList](hd: HD, tl: TL): data.Macros3.HCons[HD, TL]
[error]      |  bounds: 
[error]      |      HD := Any
[error]      |      TL >: data.Macros3.HCons[Any, data.Macros3.HList] <: data.Macros3.HList
[error]      |      HD
[error]      |      TL <: data.Macros3.HList
[error]      |      HD >: hd1
[error]      |      TL >: data.Macros3.HCons[HD, TL] <: data.Macros3.HList
[error]      |      HD
[error]      |      TL <: data.Macros3.HList
[error]      |      HD >: hd2
[error]      |      TL <: data.Macros3.HList
[error]      |  ordering: 
[error]       --------------------------------------------------------------------------

Expectation

Expect the code in #10358 to keep working on the latest compiler.

[nicolasstucki]

We do not infer the bounds for that tl. You can add explicit bound to make it work.

-      case '{HCons($h1: hd1, HCons($h2: hd2, $_ : tl))} => 
+      case '{type tl <: HList; HCons($h1: hd1, HCons($h2: hd2, $_ : `tl`))} => 

[nicolasstucki]

#18133 will fix the error message.