Streams verification

[alisevych]

Description

When a Stream is returned there should be some verification of it.
At the moment there are only expected and actual - of Object type - without assertions: see bug #1046
Probably make deepEquals comparison of the Objects inside a Stream.

Idea

Add verification of returned Stream.
When the data is generated for the input - we need to check the Stream on the output.
It's class and the objects inside.

Use cases

Functional interfaces returned from a method.

[alisevych]

@Damtev Please share your ideas on the topic

[Damtev]

Basically, there is a big difference between symbolic and concrete executions for Streams. So, we have corresponding symbolic wrappers for Streams that produce reasonable assemble models - for example, instantiating with Collection#stream() or Arrays#stream methods and then modifying with map, filter, and other methods of Stream. This model might be later reliably verified with the outer deepEquals method in code generation.

On the other hand, the concrete execution finds the real type of Stream which is java.util.stream.ReferencePipeline$Head in JDK. It could not be converted to an assemble model and, since this class is inaccessible without reflection, will be created with a lot of reflection stuff in code generation and later could not be even verified with deepEquals.

In summarizing, we need to find a way to produce an assemble model for Streams in concrete execution as it is in symbolic, or, at least, disable concrete execution for methods that return Streams as a result.

@sergeypospelov please share, what do you think