In Scala we can write the following which raises the concern how the type conversion works for Unit type.
def f(a: Unit) = println("invoked f with Unit type: " + a)
f(42)
f("temp")
The type conversion is not based on any implicit conversions defined in Predef object or is not based on sub-typing.
As per Scala Language Specification (SLS) there is an implicit conversion performed by the compiler. Excerpt form the SLS below -
Value Discarding. If e has some value type and the expected type is Unit, e is converted
to the expected type by embedding it in the term { e; () }.
The only value that Unit type can take is (). It does not represent any runtime object and is defined in the SLS section 12.2.3.
Moreover, a 0-tuple is also represented using the Unit value [SLS section 6.9]. It denotes the absence of any useful value, which makes sense since for a 0-tuple, there is no operation that seems to be useful. However, this results another type for tuples to deal with as if Tuple1, Tuple2, Tuple3.... Tuple22 wasn't enough.
PS: To generate a warning for this implicit conversion we can use scalacOptions += "-Ywarn-value-discard" in SBT configuration
