Enforce Scala Seq to use a single type only (no LUB)

Question

I would like to create and use a Seq[T] collection in Scala and ensure that it only use one type. So if I use:

val l = List(1, 2, 2.0)

a compile time error should occur - List elements should all be Double or all Int.

Answer 1

Consider -Xlint:infer-any compiler flag

Warn when a type argument is inferred to be Any

which in combination with fatal warnings at least prevents worst case LUB scenario where Any is inferred

scala -Xlint:infer-any -Werror -e 'List(42, 3.14, "picard")'
warning: a type was inferred to be `Any`; this may indicate a programming error.
List(42, 3.14, "picard")
^
error: No warnings can be incurred under -Werror.

however note this will not help if LUB is narrower than Any, for example

scala -Xlint:infer-any -Werror -e 'Vector(List(1), Set(2))'

raises no warning. Another flag that might help is -Wnumeric-widen

Warn when numerics are widened.

for example

scala -Wnumeric-widen -Xlint:infer-any -Werror -e 'def f(i: Int, d: Double): List[Double] = List(i, d)'
warning: implicit numeric widening
def f(i: Int, d: Double): List[Double] = List(i, d)
                                              ^
error: No warnings can be incurred under -Werror.