Fix #4988: Add regression test

Author: nicolasstucki

tests/pos/i4988.scala

@@ -0,0 +1,15 @@
+object Trees {
+  class PackageDef
+  class ValDef
+}
+
+class ReplCompiler {
+  type PackageDef = Trees.PackageDef
+  type ValDef = Trees.ValDef
+
+  def errors[A]: A = ???
+  def success: ValDef = ???
+
+  def typeCheck(cond: Boolean): ValDef =
+    if (cond) success else errors // failure goes away with `errors[ValDef]`
+}