Improve usability of erased values #4759
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`erased` is really the same thing as `lazy` or by-name. The semantics implies it will not be evaluated. So it is pointless to warn if an impure expression is passed to an erased val or parameter. Moreover, erased initializers are often complex expressions - after all if they were not, why erase them in the first place? These expressions are almost never recognized as pure given the weak effect checking we currently have. So the warnings are issued in the common case, which is bad.
In that sense they behave exactly like lazy vals. Making them unstable threw out most use cases. We typically set up an erased value because it contains interesting types. But to get at the types the values have to be stable in the first place!
I noticed a REPL crash because we triggered the error condition but there was no source associated with context.owner.
Flag `|` is not super-fast, so it might pay to compute this once ahead of time.
nicolasstucki
approved these changes
Jul 4, 2018
|
👍 from me (a posteriori). I thought I had found an escape, but it seems this check is even more robust than expected. However, the fact that abstract class <:<[A, B] {
type T >: A <: B
}
def cast1[A, B](x: A)(erased y: A <:< B): B = x.asInstanceOf[B] //should be safe, but isn't
def cast2[A, B](x: A)(erased y: A <:< B): B = x: y.T // error
cast1[Int, String](1)(???) // crashes
cast2[Int, String](1)(???) // error |
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Erased values were not very useful so far:
Both impediments are fixed in this PR. This is an alternate fix for #4060.