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Hi, This reads like a bunch of AI generated silliness. How on earth would you coordinate a 4 step migration process between a client and library, unless you strongly control both. /Kasper /Kasper On Tue, 24 Feb 2026 at 14: 16, Clement Cherlin</div>
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</head><body><pre style="font-family: sans-serif; font-size: 100%; white-space: pre-wrap; word-wrap: break-word">Hi,
This reads like a bunch of AI generated silliness. How on earth would
you coordinate a 4 step migration process between a client and
library, unless you strongly control both.
/Kasper
/Kasper
On Tue, 24 Feb 2026 at 14:16, Clement Cherlin <ccherlin@gmail.com> wrote:
>
> On Mon, Feb 23, 2026 at 6: 26 AM Maurizio Cimadamore <maurizio. cimadamore@ oracle. com> wrote: Hi Clement On 22/02/2026 18: 21, Clement Cherlin wrote: > Greetings, > > I have been working on the problem of generic enums off and on
>
> On Mon, Feb 23, 2026 at 6:26 AM Maurizio Cimadamore <maurizio.cimadamore@oracle.com> wrote:
>>
>> Hi Clement
>>
>> On 22/02/2026 18:21, Clement Cherlin wrote:
>> > Greetings,
>> >
>> > I have been working on the problem of generic enums off and on for
>> > quite some time (see
>> > https://github.com/Mooninaut/cursed-generic-enum-java for a previous
>> > dead-end attempt).
>> >
>> > I think I have an actual solution this time, and it doesn't require
>> > any changes to Java's type system. It only requires a slight tweak to
>> > the "extends" clause of generic enums: Replace "extends
>> > Enum<SomeEnum<T>>" with "extends Enum SomeEnum<?>>".
>>
>> I'd need more time to think about the consequences of this. That said,
>> this has a good property, in that it only affects generic enum
>> declarations. Non-generic enum would stay exactly the same.
>>
>> > There is one slightly awkward issue. It's not possible to simply pass
>> > an enum class literal to EnumSet's static factory methods without a
>> > raw cast.
>> >
>> > EnumSet.allOf((Class<Option<?>>) (Class) Option.class);
>>
>> This makes me more worried -- because this is such a common idiom that
>> (I think) has to work like for any other enum.
>>
>> In the spirit of your earlier suggestion, a possible way out would be to
>> also redefine what Foo.class mean when Foo is a generic enum -- e.g. not
>> just Class<Foo> (class-of-raw) but Class<Foo<?>> (class-of-wildcard).
>>
>> All such changes would indeed help making generic enums more useful
>> (and interoperable with EnumSet and friends), but whether all such
>> changes are actually "sound", that's a different story.
>>
>> For instance, code like:
>>
>> MyEnumSet<Option> set = MyEnumSet.allOf(Option.class)
>>
>> Would _still_ fail. You need the LHS to be _exactly_
>> MyEnumSet<Option<?>> for this to work. Which means graduating an enum
>> into a generic enum would still be a source incompatible change
>
>
>>
>> Cheers
>> Maurizio
>
>
> That's an excellent point. Doing the migration in a single step is source incompatible.
>
> However, it is possible to perform the migration in multiple steps, each step source
> compatible with the previous step, allowing for gradual migration. This would
> require multiple major releases of a library to complete. The result would be
> that at each step, clients can update the library without incompatibility, as long
> as they complete the required migration steps before upgrading to a major version
> that takes the next migration step.
>
> See https://github.com/Mooninaut/java-generic-enums/tree/main/src/main/java/org/duckdns/mooninaut/migration
> for a step-by-step example.
>
> Given a library that has an enum, which can usefully be migrated to a generic enum:
>
> Step 1:
> * Library changes method parameters of type "EnumSet<X>" (if any) to
> "EnumSet<? extends X>". This change is source-compatible for clients.
> * Casts from "EnumSet<? extends X>" to "EnumSet<X>" may be
> required inside library methods.
> * Suggestion: Some sort of @MigratingToGenericEnum annotation to prompt
> clients to update references, ideally with tool support.
>
> Step 2:
> * Clients update library, and change variables of type "EnumSet<X>" to "EnumSet<? extends X>"
> * Both "EnumSet<X>" and "EnumSet<? extends X>" compile.
>
> Step 3:
> * Library makes X generic, updates methods that return "EnumSet<X>" to
> return "EnumSet<X<?>>" This is source-compatible for clients that have completed
> step 2, with an unchecked warning on "EnumSet.allOf(X.class)" and related methods.
> * Library methods may require casts from "EnumSet<? extends X>" to "EnumSet<X<?>>"
> * Library changes "EnumSet.allOf(X.class)" to "EnumSet.allOf(X.CONSTANT.getDeclaringClass())"
> * Library removes @MigratingToGenericEnum.
>
> Step 4:
> * Clients update library, change usages of "EnumSet<? extends X>" to "EnumSet<X<?>>"
> and change "EnumSet.allOf(X.class)" to "EnumSet.allOf(X.CONSTANT.getDeclaringClass())"
> * Usages of "EnumSet<X>" now produce compilation errors.
> * Both "EnumSet<? extends X> set = EnumSet.allOf(X.class)" and
> "EnumSet<X<?>> set = EnumSet.allOf(X.CONSTANT.getDeclaringClass())" compile.
> * Tools should prompt these changes when seeing use of a generic enum with old syntax.
>
> Step 5 (optional):
> * Library replaces "EnumSet<? extends X>" with "EnumSet<X<?>>" in method parameter types.
> * Library methods can no longer be called with values of type "EnumSet<? extends X>".
> * This is source-compatible with clients that have completed step 4.
> * No further client changes are required.
> * Casts to EnumSet<X<?>> are no longer required.
> * This step is optional. The only benefit is removing remaining casts.
> Libraries can delay this step for as long as desired to allow clients time to complete step 4.
>
>
> This process is analogous to the gradual transition from a deprecated API to its replacement
> over several major versions. The non-generic syntax becomes "deprecated" at step 1, but
> still compiles, possibly with warnings, until step 4.
>
> Cheers,
> Clement
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