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, 3 insertions, 0 deletions

diff --git a/tests/pos/hk-reduce-variance.scala b/tests/pos/hk-reduce-variance.scala
new file mode 100644
index 000000000..c69777992
--- /dev/null
+++ b/tests/pos/hk-reduce-variance.scala
@@ -0,0 +1,3 @@
+class Cov[+A](a: A) {
+  def foo = Seq(a)
+}