tree 89263a8fe143eca4f70b3355e9932e74df8100e0
parent 52ae429a9e97e53c310ab5b08ad03043c9341de3
author Mike Fairhurst <mfairhurst@google.com> 1560383328 +0000
committer commit-bot@chromium.org <commit-bot@chromium.org> 1560383328 +0000

Land new subtyping algorithm; more tests but still not comprehensive.

Includes changes to implicit covariance test.

To redescribe this, for some tear-off C<T>.f, the member type may be
contravariant if T is referenced by a parameter of C<T>.f (such as if it
is of type void Function(T)). This must be a runtime failure if C<T> is
a reference which has been covariantly upcast at runtime. For example,
List<Object>.add, where the list is at runtime an instance of List<int>,
results in a static type of void Function(Object), but a runtime type of
void Function(int), which is unsound.

We omit these checks, however, when it is trivially sound for all cases
Previously, this was the case for tearing off `List<Null>.add`. This is
no longer a valid test case for NNBD where the runtime type of that list
could actually be `List<Never>`.

Therefore the test has been changed to expect `List<Null>.add` to result
in a cast, and a new NNBD test case has been added to test `List<Never>`
cases don't get a cast.

Change-Id: I61d024a249e2c2a249ad5a8037cbe3d103e3ea8b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/104361
Commit-Queue: Mike Fairhurst <mfairhurst@google.com>
Reviewed-by: Paul Berry <paulberry@google.com>