HK reduction: Remove special-case for typerefs

The special case:

  case stripped: TypeRef =>
    stripped.symbol.is(BaseTypeArg)

is wrong because you might still want to reduce applications involving
TypeRefs which are not base class parameters, like in:

  class Foo[A]
  type Alias[X] = Foo[X]
  val x: Alias[Int] = ???

`Alias` is a TypeRef so before this commit `Alias[Int]` was never reduced
to `Foo[Int]`. It should have been:

  case stripped: TypeRef if stripped.symbol.is(BaseTypeArg) =>
    true

But even this is incorrect: it assumes that we can always safely reduce HK
applications involving base class parameters, this is not the case when
the parameter kind is different from the rhs kind as illustrated by
`i1181c.scala`. We fix this by simply dropping the special case.

1 files changed, 7 insertions, 12 deletions

diff --git a/src/dotty/tools/dotc/core/TypeApplications.scala b/src/dotty/tools/dotc/core/TypeApplications.scala
index 314233709..cb11d3fdc 100644
--- a/src/dotty/tools/dotc/core/TypeApplications.scala
+++ b/src/dotty/tools/dotc/core/TypeApplications.scala
@@ -470,18 +470,13 @@ class TypeApplications(val self: Type) extends AnyVal {
       case dealiased: TypeLambda =>
         def tryReduce =
           if (!args.exists(_.isInstanceOf[TypeBounds])) {
-            val followAlias = stripped match {
-              case stripped: TypeRef =>
-                stripped.symbol.is(BaseTypeArg)
-              case _ =>
-                Config.simplifyApplications && {
-                  dealiased.resType match {
-                    case AppliedType(tyconBody, _) =>
-                      variancesConform(typParams, tyconBody.typeParams)
-                        // Reducing is safe for type inference, as kind of type constructor does not change
-                    case _ => false
-                  }
-                }
+            val followAlias = Config.simplifyApplications && {
+              dealiased.resType match {
+                case AppliedType(tyconBody, _) =>
+                  variancesConform(typParams, tyconBody.typeParams)
+                    // Reducing is safe for type inference, as kind of type constructor does not change
+                case _ => false
+              }
             }
             if ((dealiased eq stripped) || followAlias) dealiased.instantiate(args)
             else HKApply(self, args)