Merge pull request #6402 from dotty-staging/fix/i5711

Fix #5711: check that lambdas interact correctly with overloads

Author: smarter

tests/pos/i5711.scala

@@ -0,0 +1,6 @@
+object trace {
+  def apply[T](a: String, showOp: Any => String) = 0
+  def apply[T](a: Int, log: String => Unit) = 0
+
+  apply("", res => res.toString)
+}