[code-reflection] RFR: Add support for more types [v3]

[Paul Sandoz]

On Thu, 25 Apr 2024 10:56:06 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

>> This PR adds support for type-variables and wildcard type arguments in the code model `JavaType`'s hierarchy.
>> 
>> This allows the code model to reflect the source types much more accurately, as we no longer need to erase the source type at the first sign of a non-denotable type. Instead, we can use the a modified (see below) version of the `Types::upwards` function (type projection) to compute the closest **denotable** upper bound to the type found in the source code. This means that the type associated with every op in the model is a (denotable) supertype of the type in the javac AST. The fact that such type is denotable has three important consequences:
>> 
>> * the type can be expressed in the source code (in case the code model needs to be lifted back into Java source)
>> * the type must be expressible in the syntax of bytecode signature attributes (this is important e.g. for the local variable type attribute)
>> * the type can be resolved to its runtime counterpart in `j.l.r.Type` (not implemented in this PR), as explained below
>> 
>> Some parser changes were required to support this, so that we can serialize and deserialize the new types accordingly.
>> 
>> A new method has been added to `JavaType`, namely `JavaType::erasure`, which computes the erasure of a `JavaType`. This might come in handy when lowering the model into bytecode. Since supporting erasure is crucial, modelling of types has been carefully chosen as to facilitate that operation as much as possible: that is why, for example, `TypeVariableRef` contains the "principal" type-variable bound (so that we can define erasure for type-variables in a straightforward fashion, as the erasure of the primary bound).
>> 
>> #### Denotable projections
>> 
>> The code model type associated with an op result is computed by applying a modified version of `Types::upwards` - that is, the function that implements type projections as specified in [JLS 4.10.5](https://docs.oracle.com/javase/specs/jls/se22/html/jls-4.html#jls-4.10.5). The original projection algorithm is designed to leave intersection types in place - while this is handy, as it maximizes the applicability of the type inferred for local variables declared with `var`, for the code model use this is not suitable, as we'd like to get to a denotable type in the end (jshell has a similar problem, which was addressed in a more ad-hoc way).
>> 
>> It is generally possible to project an intersection type using only one of its bounds, e.g.
>> 
>> 
>> List<A & B>
>> ...
>
> Maurizio Cimadamore has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Remove whitespaces

I really like core principle of projecting upwards to a (or the nearest?) denotable supertype. It really simplifies things and is generally easy to grasp, even if the actual details can be hard to understand e.g., the set of Java types expressible in the code model is almost the same as the set of the types one can express in source code.

I agree with you having a clearer distinction for modeling type arguments, it may be useful to have a top-level Java type'ish interface covering Java type and java type argument. This seems possible, the Java type factory can create whatever instances it wants based off the type identifier information e.g.,


            if (identifier.equals("+") || identifier.equals("-")) {
                // wildcard type
                BoundKind kind = identifier.equals("+") ?
                        BoundKind.EXTENDS : BoundKind.SUPER;
                return JavaTypeArgument.wildcard(kind, typeArguments.get(0));


?

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PR Comment: https://git.openjdk.org/babylon/pull/51#issuecomment-2078057336