Reduce type lambdas even if variance changes

Previously, the added testcase failed with (when running with -Ydebug-alias):
2 |  def foo = Seq(a)
  |      ^
  |covariant type A occurs in invariant position in type => Seq.CC[Cov.this.A] of method foo

Because the type parameter of `CC` is invariant.

Of course, this is fine because `CC[A]` can be reduced to `Seq[A]`, but
before this commit, `TypeApplications#appliedTo` used to disallow
reductions that replaced an invariant type parameter with a variant one.
I believe that for type inference, only preserving the arity is
important, so I removed this restriction.

1 files changed, 4 insertions, 3 deletions

diff --git a/compiler/src/dotty/tools/dotc/core/TypeApplications.scala b/compiler/src/dotty/tools/dotc/core/TypeApplications.scala
index c713cd542..ba3e6a461 100644
--- a/compiler/src/dotty/tools/dotc/core/TypeApplications.scala
+++ b/compiler/src/dotty/tools/dotc/core/TypeApplications.scala
@@ -1,4 +1,5 @@
-package dotty.tools.dotc
+package dotty.tools
+package dotc
 package core
 
 import Types._
@@ -412,8 +413,8 @@ class TypeApplications(val self: Type) extends AnyVal {
             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
+                  sameLength(dealiased.typeParams, tyconBody.typeParams)
+                    // Reducing is safe for type inference, as kind arity of type constructor does not change
                 case _ => false
               }
             }