commit 0d636e5543a162fa0a74f8bed49d05ca3ac9044c
author Paul Berry <paulberry@google.com> Tue May 10 15:10:44 2022 +0000
committer Commit Bot <commit-bot@chromium.org> Tue May 10 15:10:44 2022 +0000
tree fd372c9405a82f783b9b8135645efb4e124cbe17
parent 51d578cf919388164de3311df7f6fab9b46eb6e2

Ensure that partial inference results aren't refined by later partial inference stages.

As part of the implementation of
https://github.com/dart-lang/language/issues/731 (improved inference
for fold etc.), I expanded the front end's type inference logic so
that instead of just having a downward phase and an upward phase, it
could have 3 or more phases.  The function that previously did
downward inference became repurposed to do "partial inference" (which
could either be the first, downward stage, or a later, horizontal
stage).  However, I failed to generalize the logic that prevents types
assigned by one inference stage from being refined by later
stages--previously this logic was only needed for upward inference,
but now it's needed for horizontal inference stages as well.  (This
logic is needed because of Dart's "runtime checked covariance"
behavior--it means that we want to stick with the type from downward
inference, even if a later horizontal inference stage is able to find
a more precise type, because that more precise type may lead to
runtime failures).

As part of this change I've re-architected the inference methods so
that they are responsible for creating and returning the list of
inferred types.  This makes the inference logic more similar between
the front end and analyzer, and is easier to read IMHO.  The total
number of list allocations is the same as before.

Change-Id: I19bfcede9c2968e50f110b571164549f16495217
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/243707
Reviewed-by: Chloe Stefantsova <cstefantsova@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>