Dropped: Weak Conformance
In some situations, Scala used a weak conformance relation when testing type compatibility or computing the least upper bound of a set of types. The principal motivation behind weak conformance was to make an expression like this have type List[Double]:
List(1.0, math.sqrt(3.0), 0, -3.3) // : List[Double]
It's "obvious" that this should be a List[Double]. However, without some special provision, the least upper bound of the lists's element types (Double, Double, Int, Double) would be AnyVal, hence the list expression would be given type List[AnyVal].
A less obvious example is the following one, which was also typed as a List[Double], using the weak conformance relation.
val n: Int = 3
val c: Char = 'X'
val d: Double = math.sqrt(3.0)
List(n, c, d) // used to be: List[Double], now: List[AnyVal]
Here, it is less clear why the type should be widened to List[Double], a List[AnyVal] seems to be an equally valid -- and more principled -- choice.
Weak conformance applies to all "numeric" types (including Char), and independently of whether the expressions are literals or not. However, in hindsight, the only intended use case is for integer literals to be adapted to the type of the other expressions. Other types of numerics have an explicit type annotation embedded in their syntax (f, d, ., L or ' for Chars) which ensures that their author really meant them to have that specific type).
Therefore, Scala 3 drops the general notion of weak conformance, and instead keeps one rule: Int literals are adapted to other numeric types if necessary.