Integrated: Initial push for type classes support

[Maurizio Cimadamore]

On Mon, 12 Jan 2026 17:58:42 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

> This PR contains support for an experimental type classes feature.
> 
>> [!WARNING]
>> Given the highly experimental nature of this work, nothing is settled in stone, and the comments below are mostly provided as a general guidance on how to use what's there.
> 
> For instance, given a record declaration like this:
> 
> 
> record MyInt(int x) { }
> 
> 
> We can define a new type class, for addition, using a regular interface, like so:
> 
> 
> interface Sum<X> {
>     X zero();
>     X add(X a, X b);
> }
> 
> And then define a _witness field_ of the type class `Sum` for the type `MyInt`, using the `__witness` keyword, like so:
> 
> 
> __witness Sum<MyInt> SUM_MYINT = new Sum<>() {
>        MyInt zero() { return new MyInt(0); }
>        MyInt add(MyInt a, MyInt b) { return new MyInt(a.x + b.x); }
> };
> 
> (For correctness, this field should be declared either in `Sum` or in `MyInt`)
> 
> Once defined, a witness can be _looked up_ using its type, like so:
> 
> 
> Sum<MyInt> sum = Sum<MyInt>.__witness;
> MyInt zero = sum.zero();
> MyInt one = new MyInt(1);
> assert sum.add(zero, one).equals(one);
> 
> 
> This prototype contains other features, such as the declaration of witness methods, and the ability to perform witness lookups at runtime. For more examples, please refer to the tests under `test/langtools/tools/javac/typeClasses`.

This pull request has now been integrated.

Changeset: 75a36f3f
Author:    Maurizio Cimadamore <mcimadamore at openjdk.org>
URL:       https://git.openjdk.org/valhalla/commit/75a36f3f912d869d5fcedce89844d4f41ce608a0
Stats:     3205 lines in 55 files changed: 3165 ins; 1 del; 39 mod

Initial push for type classes support

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PR: https://git.openjdk.org/valhalla/pull/1886