Analyzer doesn't infer type from "is" check against abstract class

[lexaknyazev]

Analyzer doesn't infer type from is check against abstract class if variable was annotated with explicit type.
https://dartpad.dartlang.org/c6d0fee76a4986d8be05e1288d288e1c

[bwilkerson]

It doesn't actually have anything to do with the fact that I is an abstract class, it has to do with the way type promotion is defined in the spec (section 16.34):

Let v be a local variable or a formal parameter. An is-expression of the form v is T shows that v has type T iff T is more specific than the type S of the expression v and both T != dynamic and S != dynamic.

But I is not more specific than A (as defined in section 19.4). Hence, we cannot promote the type here and this is working as expected.

[lexaknyazev]

So why in the code snippet above a1 is I evaluates to true, if I is not more specific than A?

[bwilkerson]

Because of the difference between analysis-time (equivalent to compile-time) and run-time semantics. When we're performing a static analysis of the code we don't try to do full flow analysis. You told us that a1 could be any kind of A and we don't try to figure out that it will always be a B at this point in the code. But at run time the VM knows that the value of a1 it's an instance of B and hence also an instance of I.

[lexaknyazev]

Thanks. Is it a correct way to suppress such analyzer warnings?

 if (a1 is I) (a1 as Object/*=I*/).foo();

[bwilkerson]

Nope. Try putting that into DartPad; you'll see two errors rather than one.

There are three ways to disable the warning. The best is to add an explicit cast to the tested for type:

if (a1 is I) (a1 as I).foo();

The other two involve telling analyzer not to type check your code (which I don't recommend). One is to replace the line with the warning with:

if (a1 is I) (a1 as dynamic).foo();

The other is to change the declaration to:

var a1 = new B();

[lexaknyazev]

Try putting that into DartPad; you'll see two errors rather than one.

DartPad and IntelliJ don't give any errors with strong mode enabled. Ctrl-click on foo() in IntelliJ navigates correctly to I.foo().

[bwilkerson]

Correct. That's because, with strong mode enabled, the code is effectively rewritten to

if (a1 is I) (a1 as I).foo();

Without strong mode the content of the comment is ignored. But there's no reason to make the code valid only when strong mode is enabled :-)