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b279238e48
In the following expression types, the static type is computed using
the least upper bound ("LUB") of their subexpressions (adjusted as
appropriate to account for the null-shorting behaviors of `??` and
`??=`):
- Conditional expressions (`a ? b : c`)
- If-null expressions (`a ?? b`)
- If-null assignments (`a ??= b`)
- Switch expressions (`switch (s) { p0 => e0, ... }`)
This can lead to problems since the LUB computation sometimes produces
a greater bound than is strictly necessary (for example if there are
multiple candidate bounds at the same level of the class hierarchy,
the LUB algorithm will walk up the class hierarchy until it finds a
level at which there is a unique result). For a discussion of the kind
of problems that can arise, see
https://github.com/dart-lang/language/issues/1618.
This change improves the situation by changing the analysis of these
four expression types so that after computing a candidate static type
using LUB, if that static type does not satisfy the expression's
context, but the static types of all the subexpressions *do* satisfy
the expression's context, then the greatest closure of the context is
used as the static type instead of the LUB. This is the algorithm
proposed in
https://github.com/dart-lang/language/issues/1618#issuecomment-1507241494.
This is theoretically a breaking change (since it can change code that
demotes a local variable into code that doesn't, and then the demotion
or lack of demotion can have follow-on effects in later code). So it
is implemented behind the `inference-update-3` experiment
flag. However, in practice it is minimally breaking; a test over all
of google3 found no test failures from turning the feature on.
Since one of these expression types (switch expressions) is
implemented in `package:_fe_analyzer_shared`, but the other three are
implemented separately in the `package:analyzer` and
`package:front_end`, this change required modifications to all three
packages. I've included tests for the new functionality, following the
testing style of each package. I've also included a comprehensive set
of language tests that fully exercises the feature regardless of how
it's implemented.
Since `package:front_end` has many different implementations of `??=`
depending on the form of the left hand side, I've tried to be quite
comprehensive in the language tests, covering each type of assignable
expression that might appear to the left of `??=`.
Change-Id: I13a6168b6edf6eac1e52ecdb3532985af19dbcdf
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/353440
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Reviewed-by: Chloe Stefantsova <cstefantsova@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>
Reviewed-by: Erik Ernst <eernst@google.com>
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| .. | ||
| co19 | ||
| corelib | ||
| dartdevc | ||
| ffi | ||
| language | ||
| lib | ||
| modular | ||
| standalone | ||
| web | ||
| legacy_status_dart2js.csv | ||
| OWNERS | ||
| README.md | ||
This directory contains tests of the language and core library implementations. For more information, see https://github.com/dart-lang/sdk/wiki/Testing.