Fix argDenot, widenDealias before baseType, to get the right type arguments

When looking to select the type member Y on the class Foo,
if the prefix is `(a : (([O] =>> Foo[O, S]) @UV)[Int])` then you could say
it's correct for the basetype of that type with class Foo to be
`(([O] =>> Foo[O, S]) @UV)[Int]`, however if you then try to access the
type arguments, it would be incorrect to end up looking at `[Int]`,
rather than `[Int, S]`.

I went with fixing the input before baseType - the alternative would be
to make baseType do the same type constructor dealiasWidening.

I also considered making `isArgPrefixOf` return false, but broke a
handful of tests, so I went back to argDenot.

Author: dwijnand

compiler/src/dotty/tools/dotc/core/Types.scala

@@ -2402,7 +2402,7 @@ object Types {
 
     private def argDenot(param: TypeSymbol)(using Context): Denotation = {
       val cls = param.owner
-      val args = prefix.baseType(cls).argInfos
+      val args = prefix.widenDealias.baseType(cls).argInfos
       val typeParams = cls.typeParams
 
       def concretize(arg: Type, tparam: TypeSymbol) = arg match {

compiler/test/dotc/pos-test-pickling.blacklist

@@ -21,6 +21,7 @@ i15181.scala
 i15922.scala
 t5031_2.scala
 i16997.scala
+argDenot-alpakka.scala
 
 # Tree is huge and blows stack for printing Text
 i7034.scala

tests/pos/argDenot-alpakka.min.scala

@@ -0,0 +1,9 @@
+import scala.annotation.unchecked.uncheckedVariance as uV
+
+trait Test:
+  def test[S] =
+    val a: (([O] =>> Foo[O, S]) @uV)[Int] = ???
+    a.m()
+
+class Foo[X, Y]:
+  def m(): Y = ???

tests/pos/argDenot-alpakka.scala

@@ -0,0 +1,21 @@
+import scala.annotation.unchecked.uncheckedVariance as uV
+
+trait Test:
+  def split[I, M](in: Flow[I, Byte, M]): SubFlow[Byte, M, in.Repr]
+  def test =
+    split(new Flow[Int, Byte, Unit])
+      .via[Char]
+      .merge
+      .filter()
+
+trait FlowOps[+Out, +Mat]:
+  type Repr[+O] <: FlowOps[O, Mat] { type Repr[+O] = FlowOps.this.Repr[O] }
+  def via[O]:   Repr[O]   = ???
+  def filter(): Repr[Out] = ???
+
+class Flow[-In, +Out, +Mat] extends FlowOps[Out, Mat]:
+  type Repr[+O] = Flow[In @uV, O, Mat @uV]
+
+class SubFlow[+Out, +Mat, +F[+_]] extends FlowOps[Out, Mat]:
+  type Repr[+O] = SubFlow[O, Mat @uV, F @uV]
+  def merge: F[Out] = ???