commit 0abd2f3f7599ffb3b8320cf29fee374925a51b6e
author Paul Berry <paulberry@google.com> Wed Mar 20 18:49:26 2024 +0000
committer Commit Queue <dart-scoped@luci-project-accounts.iam.gserviceaccount.com> Wed Mar 20 18:49:26 2024 +0000
tree 5464ba30e201bde3f424f6708aaafa7314264ab4
parent f7d5d0cfbeaade73f322811da5e1527cd053201f

Make null contexts impossible in analyzer resolution.

The analyzer's mechanism for passing contexts down the stack while
recursively resolving expressions is to add an optional `contextType`
parameter to some of the `visit` methods in `ResolverVisitor`. This
parameter is only included in `visit` methods that would actually use
it (e.g. `visitDoubleLiteral` doesn't have it, since the analysis of a
double literal doesn't depend on the context).

When the resolver needs to analyze a subexpression, if it needs to
supply a context, it uses `ExpressionImpl.resolveExpression` to
dispatch to the appropriate `visit` method; this passes the context
along to the optional `contextType` parameter. If, on the other hand,
it **doesn't** need to supply a context, it uses the standard
AstVisitor mechanism, which dispatches to the `visit` method without
supplying a context type, so the `contextType` parameter takes on its
default value.

Previous to this CL, the default value of each `contextType` parameter
was `null`; this had the unintended consequence of making a
distinction between a `null` context and a context of `_`
(`UnknownInferredType.instance`). The front end, by contrast, has a
visitor paradigm that allows passing a required argument through to
the `visit` method, so its context parameters are all non-nullable,
and it makes no such distinction.

Prior to addressing language issue 3648
(https://github.com/dart-lang/language/issues/3648), the difference
was only observable for `await` expressions. Now that that issue has
been addressed, there is no user-visible difference between `_` and
the null context. But it's easy to imagine a difference accidentally
sneaking in during future development.

To avoid future bugs, this change makes `_` the default value for each
`contextType` parameter. To do this, it was necessary to change
`UnknownInferredType.instance` from a final variable to a const.

To the best of my knowledge, this change should have no user-visible
effect.

Bug: https://github.com/dart-lang/language/issues/3648
Change-Id: Id74a513366831239df2d56ceac57c2dbe5c5084e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/357522
Commit-Queue: Paul Berry <paulberry@google.com>
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>