Paul Berry
3d01b0aac3
Since matching a constant pattern implicitly performs an equality
check, we do the same flow analysis for constant patterns that we
would have done for an explicit equality test. There are only two
user-visible behaviours:
- If the constant pattern is a `null` literal, then in the code path
where the pattern fails to match, the scrutinee is promoted to a
non-nullable.
- If the constant pattern and the scrutinee are both `null` literals,
the pattern match is known to always succeed (so the code path where
the pattern fails to match is marked as unreachable).
The first of these two behaviours is genuine useful to the user, since
it allows things like:
switch (expr) {
case null:
...
...
default:
// expr is known to be non-`null`.
}
The second behaviour is not so useful, but seemed worth doing to
maximize code sharing and to keep the behaviour consistent between
pattern matching and explicit null checks.
Making this work required extracting some of the logic that was
formerly in `_FlowAnalysisImpl.equalityOperation_end` into a method
`_equalityCheck`, which understands how to analyze an equality check
regardless of whether it's an explicit equality expression or an
implicit part of a pattern. It returns an `_EqualityCheckResult`,
which `equalityOperation_end` examines to determine exactly what needs
to be done to for an equality test in an expression context;
similarly, `_FlowAnalysisImpl.constantPattern_end` now calls
`_equalityCheck` and then does the right thing for patterns.
It was also necessary to extract the logic from
`nullCheckOrAssertPattern_begin` for handling null checks in patterns;
that logic is now in `_nullCheckPattern`, which is also called by
`constantPattern_end`.
In the process of testing this change, I discovered (and fixed) a
minor bug: when analyzing a switch statement or switch expression, we
were re-capturing the value of the scrutinee before visiting each
pattern. This meant, for example, that we would analyze the following
code incorrectly:
int? i = ...;
switch (i) {
case _ when f(i = ...):
...
case null:
...
default:
// `i` should *not* be promoted to non-null here, because it's
// not guaranteed to be the same as the value matched by `case
// null` above.
}
Bug: https://github.com/dart-lang/sdk/issues/50419
Change-Id: I4d30d6bc2673d9968e69f3384a37e1b2b9cc4a8f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/280861
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>
FE/analyzer shared code
This package contains logic that is shared between the front_end and analyzer packages. It is intended solely to facilitate development of the Dart SDK, and is not intended for use by end users. In particular, this package has no public API, so no guarantee is made of compatibility between one version of the package and the next.
End users should consider using the analyzer package to analyze Dart source code.