I suspect that when the list of type preserving selectors was created the first/last setters either didn't exist or were just overlooked. Any dynamic List could have its inferred type changed by these operations.
Due to Dart2js's type representation this bug affects more than just dynamic lists. We track some simple values as part of the type system. So an operation that modifies a value can technically modify the type, as Dart2js represents it, even if the "real" type is preserved.
In the added test we would represent the list literal's type as "List(length: 2, elementType: Bool(true))". Notice the type states the elementType is specifically true, not just bool. Thus the first/last operations are modifying that type. But since we aren't registering this type change, SSA optimizes away the call/index and inlines the element itself.
Interestingly, this primarily manifested for bools due to a check in SSA:
https://github.com/dart-lang/sdk/blob/main/pkg/compiler/lib/src/ssa/optimize.dart#L475
In that code we are inlining constant-like expressions for the arguments of static invocations (such as the argument to a Expect.isTrue call). However, a few lines earlier you will see we only inline bool constants:
https://github.com/dart-lang/sdk/blob/main/pkg/compiler/lib/src/ssa/optimize.dart#L467
So when the list element type is anything other than a bool, this inlining does not trigger thus avoiding the bug.
Bug: https://github.com/dart-lang/sdk/issues/53944
Change-Id: I4e893903f335fc99b13cf526736c27bb066a4bad
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/334420
Reviewed-by: Stephen Adams <sra@google.com>
Commit-Queue: Nate Biggs <natebiggs@google.com>
Nate Biggs
9eba05482b