That spelling is problematic. Never use!
?= will be interpreted instead as ? = when used without surrounding whitespace.
var x: Int? = nil
x? = 1 // This is rather obviously not using the operator.
x?=1 // This is also not using the operator, but that's not obvious.
#expect(x == nil)
x ?= 1 // This is using the operator.
#expect(x == 1)