Fix #2840: Add regression test

Author: allanrenucci

compiler/src/dotty/tools/dotc/transform/PatternMatcher.scala

@@ -969,7 +969,7 @@ object PatternMatcher {
         }
         def numConsts(cdefs: List[CaseDef]): Int = {
           val tpes = cdefs.map(_.pat.tpe)
-          tpes.toSet.size: Int // without the type ascription, testPickling fails because of #2840.
+          tpes.toSet.size
         }
         if (numConsts(resultCases) < numConsts(original.cases))
           ctx.warning(UnableToEmitSwitch(), original.pos)

tests/pickling/i2840.scala

@@ -0,0 +1,8 @@
+object Test {
+  class Type
+  def f(x: Type): Type = ???
+  def g(xs: List[Type]): Int = {
+    val ys = xs.map(f)
+    ys.toSet.size
+  }
+}