feat(ast): add support for LambdaExpr to infer type from return expr type

[zhumin8]

This will enable it to be assigned to an appropriate variable

Before this change: cannot assign a lambda expression to a variable because its type is always VOID.
After this change: lambda expression builder will infer type from returnExpr’s expression type. Allowing it to be assigned to an appropriate variable. Because returnExpr is a required field, its type should always be accessible.
Example: a naive sample usage is provided in JavaWriterVisitorTest.java as test. Another example is what I am trying to generate for spring autoconfig code: this.projectIdProvider = () -> clientProperties.getProjectId();. Ideally through method reference, but lambda assignment does the same thing and it’s an easier feature addition.

[blakeli0]

Looks a good feature improvement. If I'm understanding correctly, we cannot assign a lambda expression to a variable before this change because the return type is always void, so it would error out on checking the type of the left hand variable and the right hand lambda expression?
Can you please provide a little more description to the PR and maybe some examples?

[zhumin8]

Yes, the improvement is exactly as you described.
I've updated the description above to include a bit more explanation.

[zhumin8]

keeping this despite sonar complains about already defined in Expr interface. It's seems the recommend way for @autovalue as explained in this doc. Also keeps consistent with other expression classes like ValueExpr, CastExpr

[blakeli0]

You might already considered it, can we just implement the type() method to return the type of ReturnExpr, like return returnExpr().expr().type()? So that we don't have to change any of the code in the builder.

[zhumin8]

hmm, I actually did not think of that. Sounds like a better idea with less changes to existing code. The only thing is we won't be able to set a default value for type(), but that should be fine because ReturnExpr().expr() should always have type associated.
Let me make the changes.

[blakeli0]

If you want, we can still expose the setType() method and set a default value in the builder() method, but like you said ReturnExpr().expr() should always have type associated so we should never call setType() explicitly.

[sonarcloud]

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[blakeli0]

LGTM