Crash when extending a type alias to a trait with implicit parameter

[Medowhill]

I found this issue while trying to write some tests for issue #4582. PR #4827 is related.

trait T[X]
type Foo[X, Y] = T[X]
class D[X] extends Foo[X, Unit]

trait T[X: Numeric]
type Foo[X] = T[X]
class D[X: Numeric] extends Foo[X]

Two above examples were compiled.

trait T[X: Numeric]
type Foo[X, Y] = T[X]
class D[X: Numeric] extends Foo[X, Unit]

However, this one resulted the following crash:

Exception in thread "main" java.lang.AssertionError: assertion failed: missing parameters for trait T from ($outer: C, implicit evidence$2: scala.math.Numeric) extends Object() with T {
  private implicit val evidence$2: scala.math.Numeric
  private val $outer: C
  private <accessor> def $outer(): C = this.$outer
  final def C$D$$$outer(): C = D.this.$outer()
} should have been caught in typer
	at scala.Predef$.assert(Predef.scala:219)
	at dotty.tools.dotc.transform.Mixin.nextArgument$1(Mixin.scala:209)
	at dotty.tools.dotc.transform.Mixin.$anonfun$transformTemplate$12(Mixin.scala:235)
	at scala.collection.TraversableLike$WithFilter.$anonfun$map$2(TraversableLike.scala:739)
	at scala.collection.immutable.List.foreach(List.scala:389)
	at scala.collection.TraversableLike$WithFilter.map(TraversableLike.scala:738)
	at dotty.tools.dotc.transform.Mixin.traitInits$1(Mixin.scala:217)
	at dotty.tools.dotc.transform.Mixin.$anonfun$transformTemplate$17(Mixin.scala:263)
	at scala.collection.immutable.List.flatMap(List.scala:335)
	at dotty.tools.dotc.transform.Mixin.transformTemplate(Mixin.scala:262)
	at dotty.tools.dotc.transform.Mixin.transformTemplate(Mixin.scala:98)
	at dotty.tools.dotc.transform.MegaPhase.goTemplate(MegaPhase.scala:922)

[Medowhill]

Further investigation

1st code:

// after frontend
class D[X >: Nothing <: Any]() extends Object() with C.this.Foo[D.this.X, Unit] {
// after erasure
class D($outer: C) extends Object() with T {

2nd code:

// after frontend
class D[X >: Nothing <: Any](implicit evidence$2: Numeric[X])
  extends Object() with T[D.this.X]()(evidence$2) {
// after erasure
class D($outer: C, implicit evidence$2: scala.math.Numeric) extends Object() with T(evidence$2) {

3rd code:

// after frontend
class D[X >: Nothing <: Any](implicit evidence$2: Numeric[X])
  extends Object () with C.this.Foo[D.this.X, Unit] {
// after erasure
class D($outer: C, implicit evidence$2: scala.math.Numeric) extends Object() with T {

and another crash

trait T(x: Int)
type Foo[X] = T
class D extends Foo[Int]

It should be rejected by the typer but it passed the phase and resulted the same crash.

[Medowhill]

I think changing Typer.scala as the following would be a valid fix for this issue.

diff --git a/compiler/src/dotty/tools/dotc/typer/Typer.scala b/compiler/src/dotty/tools/dotc/typer/Typer.scala
index da66c5b62..c37d0d419 100644
--- a/compiler/src/dotty/tools/dotc/typer/Typer.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Typer.scala
@@ -1475,7 +1475,7 @@ class Typer extends Namer
      */
     def maybeCall(ref: Tree, psym: Symbol, cinfo: Type): Tree = cinfo.stripPoly match {
       case cinfo @ MethodType(Nil) if cinfo.resultType.isImplicitMethod =>
-        typedExpr(untpd.New(ref, Nil))(superCtx)
+        typedExpr(untpd.New(untpd.TypedSplice(ref), Nil))(superCtx)
       case cinfo @ MethodType(Nil) if !cinfo.resultType.isInstanceOf[MethodType] =>
         ref
       case cinfo: MethodType =>
@@ -1492,7 +1492,7 @@ class Typer extends Namer
 
     def typedParent(tree: untpd.Tree): Tree = {
       var result = if (tree.isType) typedType(tree)(superCtx) else typedExpr(tree)(superCtx)
-      val psym = result.tpe.typeSymbol
+      val psym = result.tpe.dealias.typeSymbol
       if (seenParents.contains(psym) && !cls.isRefinementClass)
         ctx.error(i"$psym is extended twice", tree.pos)
       seenParents += psym

However, it needs a correct implementation of untpd.New. Therefore, I'll make a PR after #4798 is merged to the master branch.

[allanrenucci]

However, it needs a correct implementation of untpd.New. Therefore, I'll make a PR after #4798 is merged to the master branch.

You can make a PR based on the #4798 branch and rebase once it's merged. Or wait, up to you! 😄