What is the concept of “weak conformance” in Scala?

Question

I just recently encountered the term \"Weak Conformance\" (in Stack Overflow user retronym\'s answer to How to set up implicit conversion to allow arithmetic between num

Answer 1

According to Scala lang spec 2.8:
http://www.scala-lang.org/archives/downloads/distrib/files/nightly/pdfs/ScalaReference.pdf

3.5.3 Weak Conformance
In some situations Scala uses a more genral conformance relation. A type S weakly conforms to a type T , written S <:w T , if S <: T or both S and T are primitive number types and S precedes T in the following ordering.
Byte <:w Short
Byte <:w Character
Short <: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.