Move Null to the bottom of nullable types

[floitschG]

In Dart 1, dynamic was equivalent to both top (aka Object) and bottom (subclass of every type). In strong mode, dynamic only stands for Object (+ dynamic calls). This means that a List<dynamic> is not a subclass of List<int> anymore.

Usually, there are ways to replace dynamic with a better type, but in some cases, one would actually want to provide a type that is a subtype of all other types. A very common one is, for example, const <Bottom>[] where the constant empty list would like to be used for any other list.

Since, in Dart1, all types are nullable, the null value dynamically already has the bottom type. With this bug we also change the Null type to be at the bottom of the typing hierarchy: Null <: A for every A. This makes sense: List<Null> is a List<int> (since null is an int).

Once we have nullable types we should introduce a non-nullable version of Null (called Nothing). See #27488

• spec Move Null to the Bottom - spec #28028
• analyzer Move Null to the bottom in the Analyzer #28027
• dart2js Move Null to the bottom in dart2js #28026
• vm Move Null to the Bottom (VM) #28025
• Make sure changelog has an entry for this

[a-siva]

Can we add some negative and positive tests for this feature so that we can verify the implementations.

[mit-mit]

Spec is complete, moving this to 1.22 for implementation.

[mit-mit]

Siva wrote:

Can we add some negative and positive tests for this feature so that we can verify the implementations.

@floitschG do we have some shared tests in place?

[lrhn]

I have landed a test (language/null_bottom_test) and Johnni has landed some too when he made the dart2js implementation (null_is_bottom{,_type}_test).

If you find something that isn't covered, please say so, and I'll add to the tests.

[leafpetersen]

Issue to clarify what methods are on Null and bottom here:

#28430

[mit-mit]

Marking dart2js and vm done

[mit-mit]

Marking Analyzer done

[mit-mit]

Assigning to Florian for getting changelog updated.