[Constants-Phase1] Analyzer Issue for Constants Implementation

[JekCharlsonYu]

Request: As per implementation plan,

***Analyzer and CFE
**The analyzer and CFE implements support for the new constant and potentially constant expressions behind the experimental flag.

Phase 1 ETA: please see issue [tracker] (dart-lang/language#60)

Tasks:

• Cast Operator
• Type Test Operator
• Equality Operator
• The operator >>>
• The operator &
• The operator |
• The operator ^
• Short-Circuit &&
• Short-Circuit ||
• Short-Circuit ??
• Short-Circuit ?:
• Fix Summary representation of constants does not reflect the results of type inference #33441
• Address co19 failures
• Verify that no test failures occur if IsEnabledByDefault.constant_update_2018 is set to true
• Add an SDK constraint hint Analyzer should warn when using constant-update-2018 features with an SDK constraint that does not guarantee support #36296

[bwilkerson]

@lrhn @mit-mit

I started looking at what it would take to implement this experiment, and I found that some of the work is already done. For example, the && and || operators have been lazy in the analyzer since 1/30/18 (https://dart-review.googlesource.com/37540).

We could potentially put the current implementation behind the flag and then revert the change to make them eagerly evaluate their arguments, but I'm concerned that that would break existing users.

Are we certain that all of these changes are really different than the 2.1 behavior?

How would you like me to handle this situation?

[bwilkerson]

The rules for equality ('==' and '!=') were updated on 8/23/18 (https://dart-review.googlesource.com/71340), so they are also already in 2.1.

[lrhn]

It seems that == and != works everywhere (analyzer, dart2js, VM),
but && and || only works in the analyzer and VM, they fail in dart2js.
In the VM, anything goes, really, even things that were never valid. It's probably a case of CFE missing errors, where dart2js has its own constant system catching the error, and running the old rules.

It does mean that there might be code out there using these features, passing the analyzer and running on the VM, so we can't retract the already working functionality.
So, ==, !=, && and || functionality should keep working without the experimental flag in the analyzer and VM (CFE). Dart2js is free to keep it not working without the experimental flag, or just follow the CFE.

[bwilkerson]

Thanks!

@sigmundch (For the dart2js reference in the previous comment.)

[stereotype441]

Note that when people start using the new cast and type test operators, they are likely to run into trouble due to #33441, because the analyzer doesn't always assign correct types to constants that are maps or lists. Accordingly, I'm adding a bullet item to fix #33441 before we consider the implementation complete.

[MichaelRFairhurst]

#33441 has been closed, the real issue behind it is #35441, which will not affect this issue to nearly the same degree.

I'm still hoping to solve #35441 this week, but its not in my opinion a blocker.

[stereotype441]

We've now completed all the implementation work we know about, but the co19 tests aren't passing, so we're investigating that.

[stereotype441]

Note: there are some new co19 failures (co19_2/LanguageFeatures/Constant_update2018/...) on bot analyzer-linux-release, analyzer-mac-release, and analyzer-win-release as of today.