Fix scala#3081: Add regression test

Author: nicolasstucki

tests/neg/i3081.scala

@@ -0,0 +1,10 @@
+object Test {
+  inline def encode(n: String): List[String] =
+    for {
+      a <- List("s")
+      b <- List("w")
+      c <- encode(n)
+    } yield c
+
+  def encode0(n: String) = encode(n) // error
+}