commit f58adc5dc3e9ecd9186771a1e62df84372cbcbe4
author Paul Berry <paulberry@google.com> Tue Mar 15 22:00:22 2022 +0000
committer Commit Bot <commit-bot@chromium.org> Tue Mar 15 22:00:22 2022 +0000
tree 06a9f98af8c4ffd49d92c282acc13de5a2c5d8f7
parent b6c5e52af6771762aa593b333fd1185f66674658

Rework heuristic for "fixing" type parameters after downwards inference.

Sometimes the upwards inference phase of generic type inference is
capable of assigning a more specific type than the downwards inference
phase, but we don't want to use the more specific type due to Dart's
"runtime checked covariant generics" design.  For example, in this
code:

    List<num> x = [1, 2, 3];
    x.add(4.0);

Downwards inference provisionally considers the list to be a
`List<num>`.  Without this heuristic, upwards inference would refine
the type to `List<int>`, leading to a runtime failure.  So what we do
is "fix" the type parameter to `num` after downwards inference,
preventing upwards inference from doing any further refinement.

(Note that the heuristic isn't needed for type parameters whose
variance is explicitly specified using the as-yet-unreleased
"variance" feature, since type parameters whose variance is explicitly
specified don't undergo implicit runtime checks).

Previously, this heuristic was implemented in a kludgy way: we would
go ahead and gather type constraints during both the upwards and
downwards inference phases, and then just prior to choosing the final
type, if the downward constaints were sufficient to lock down the
type, we would discard the constraints from the upwardsr inference
phase.

This change simplifies the logic so that we figure out which
parameters to fix after the downwards inference phase, and simply stop
accumulating constraints for them.

This is a preparatory step towards implementing
https://github.com/dart-lang/language/issues/731, which will require
splitting type inference into more phases.

Change-Id: I0cc5774e61a5ace589ea0f45a20fbc1a67c8b940
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/237381
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>