Nullary and Nilary Methods

Language Design
4

That is not possible, as they have the same erasure. You can build an intersection type, in Scala 2 using with, then linearization kicks in

scala> class C { def f = 0 }
scala> class D { def f() = 1 }

scala> def m(x: C with D) = x.f() // nilary
scala> def m(x: D with C) = x.f   // nullary

For Scala 3 (C & D vs D & C) I’m not sure exactly what’s the spec, but it seems the order also matters (Fix #7425: Handle nullary/parameterless conflicts in infoMeet · dotty-staging/dotty@20818af · GitHub).

Yes, you have to invoke f without (), or provide the argument explicitly. The compiler infers implicit arguments when an entire implicit argument list is missing (and doesn’t go into eta-expansion if no implicit can be found).

For default arguments, the opposite is true: they are only used when an argument list is present, but incomplete.

scala> def f(x: Int = 1) = x

scala> f // eta-expansion in 2.14 and Scala 3
val res0: Int => Int = Lambda$1366/113202956@6c9a3661

scala> f()
val res1: Int = 1
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