Strong mode analyzer incorrectly infers return type of ??=

[charliereams]

The following code should be strong-mode compliant:

class X {}

UnmodifiableListView<X> a = new UnmodifiableListView<X>([]);
Iterable<X> b = null;
Iterable<X> c = (b ??= a);

but the analyzer actually produces the error:

#Unsound implicit cast from Object to Iterable<X> [STRONG_MODE_DOWN_CAST_COMPOSITE]

Note that this very similar (and functionally equivalent) code works fine:

class X {}

List<X> a = new UnmodifiableListView<X>([]);
Iterable<X> b = null;
Iterable<X> c = (b ??= a);

so it seems it's specifically having trouble working out the common subtype of Iterable and UnmodifiableListView. Maybe mix-in related?

[leafpetersen]

I believe that this is an instance of this:

#25821

We are currently using the spec mode LUB, which is not very useful:

[leafpetersen]

For discussion of spec mode LUB see:

#19282
#19425

[leafpetersen]

Closing as duplicate of #25821 and #25802 , please re-open if you think something else is going on.