Analyzer should interpret a missing sdk lower constraint as 2.7

[leafpetersen]

Per language team discussion (summarized briefly here we are changing the interpretation of a pubspec sdk constraint with a missing lower bound (or in general no sdk constraint at all, though that will become a pub error) to imply a language version of 2.7. I believe that the analyzer is one of the few tools that reads pubspec.yaml directly, and hence this will need to be changed there.

This is blocker for beta, and will need to be cherry picked if it doesn't make the beta branch cut.

cc @scheglov @devoncarew

cc @pcsosinski @mit-mit @franklinyow

[devoncarew]

We offer lints based on the pubspec file, but I believe that we take the .dart_tool/package_config.json info as the system-of-record wrt package language versioning. I suspect there's no work to do here for the analyzer, but @scheglov can confirm.

@pq - do we lint if there's no lower bound constraint (i.e., a constraint of just <=3.0.0)? If no, we should open an issue for a related lint.

[pq]

@pq - do we lint if there's no lower bound constraint (i.e., a constraint of just <=3.0.0)? If no, we should open an issue for a related lint.

tracked: #44054

[scheglov]

My understanding is that in the past, when there is no the lower bound constraint for SDK, pub would generate no languageVersion field for this package in .dart_tool/package_config.json, and we used the language version of the current SDK since 77f2019.

So, if now we decide that in the absence of the lower bound constraint for SDK pub will generate 2.7 in the languageVersion field, then the analyzer has almost nothing to do. The only thing I can think about is maybe that the analyzer should also use 2.7 instead of the current SDK language version, for cases when the analyzer is used as a package, and an old version of pub was used that did not have this 2.7 generating change. @leafpetersen do we need such backward support?

[bwilkerson]

Do we need to change the default in cases where there is no package_config.json?

[leafpetersen]

The only thing I can think about is maybe that the analyzer should also use 2.7 instead of the current SDK language version, for cases when the analyzer is used as a package, and an old version of pub was used that did not have this 2.7 generating change. @leafpetersen do we need such backward support?

I don't think so.

Do we need to change the default in cases where there is no package_config.json?

No. Files that are not part of a package continue to be opted in by default.

@jakemac53 Do we know of any other ways the analyzer is affected? If not, we can close this out.

[jakemac53]

Nothing I am aware of, build_runner does create a package config manually and pass it to the analyzer but we also generate that from the existing package config and not pubspecs, so as long as pub is writing a number into the package config it should be good to go.

[devoncarew]

Closing as I don't believe there's anything for the analyzer to do here.