The new logic classifies the LHS and RHS of the equality check as
either null, non-nullable, or potentially nullable, and then either
promotes, treats the expression as equivalent to a boolean, or does
nothing, as appropriate.
This means, for example, that a comparison between a non-nullable
value and `null` is now known to evaluate to `true` for reachability
analysis.
Note: as part of this change, it was tempting to trigger promotion
whenever a varialbe is equality compared to an expression of type
`Null`, but this would be unsound (consider `(int? x) => x == (x =
null) ? true : x.isEven`). So we still only promote when the variable
is compared to a literal `null`.
Fixes#41985.
There's a corresponding spec change out for review:
https://github.com/dart-lang/language/pull/1134
Change-Id: Id7f1d4eaa3b0fa57124445bb8352eef32c304feb
Bug: https://github.com/dart-lang/sdk/issues/41985
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/155926
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Paul Berry
e403e6ae77