Weak Conformance
In some situations Scala uses a more general conformance relation. A type S weakly conforms to a type T, written S<:wT, if S<:T or both S and T are primitive number types and S precedes T in the following ordering.
Byte <:w Short
Short <:w Int
Char <:w Int
Int <:w Long
Long <:w Float
Float <:w Double
A weak least upper bound is a least upper bound with respect to weak conformance.