Confusing Analyzer output when package language version is too old for null safety

[natebosch]

In a package where the language version in package_config.json is < 2.9 and a library with a // @dart = 2.9 comment, running dartanalyzer --enable-experiment=non-nullable . prints:

  error • This requires the 'non-nullable' language feature to be enabled. • foo.dart:10:20 • experiment_not_enabled
1 error found.

It would be less confusing if the error message said something about not being able to opt in to a newer language version with a comment than what is in the package_config.json. Or potentially even better, if it let you use the experiment anyway. This is on a recent dev SDK.

[natebosch]

See also #42323 where the CFE/VM have an output which is also confusing.

[scheglov]

@leafpetersen Is behavior of // @dart = specified anywhere?

Currently in analyzer we start with some set of language features, default, or with enabled experiments; and then this set passes through two phases of restricting - the first time based on the package config, and the second time based on the // @dart override.

So, if we specify a higher language version than for the whole package, it is no-op, the features have already been removed from the set.

It seems right from the perspective of migrating a package library by library.

[leafpetersen]

@leafpetersen Is behavior of // @dart = specified anywhere?

Here

And more specifically here

[scheglov]

Great, thank you!

[scheglov]

https://dart-review.googlesource.com/c/sdk/+/151423

[scheglov]

One more related fix 60bb0d3.