なぜambiguousなのだろう？

$ swift Welcome to Apple Swift version 4.1 (swiftlang-902.0.43 clang-902.0.37.1). Type :help for assistance. 1> Optional(1).map(dump) error: repl.swift:1:17: error: ambiguous reference to member 'dump(_:to:name:indent:maxDepth:maxItems:)' Optional(1).map(dump) ^~~~

(edited)