[lworld] RFR: JDK-8247663: [lworld] LambdaMetaFactory should allow an inline value projection type to convert to its reference projection type

[Mandy Chung]

On Wed, 8 Jul 2020 18:17:55 GMT, Roger Riggs <rriggs at openjdk.org> wrote:

>> This patch fixes `LambdaMetaFactory` to allow inline value projection type to convert to its reference projection type.
>> 
>> `Class::isAssignableFrom` and `Class::asSubclass` are the APIs to determine if a given type can be safely cast to this
>> Class.
>> -  Class::isAssignableFrom tests if a given type can be converted to this Class via a widening reference conversion.
>> - Class::asSubclass tests if a given type can be converted to this Class via a narrowing conversion.
>> 
>> The current thought is to keep  `Class::isAssignableFrom` and `Class::asSubclass` to respect the class file hierarchy
>> and a cast is needed when converting from `V.val` to `V.ref`.   `LambdaMetaFactory` and other frameworks will need to
>> extend the type adaptability check to support inline types.  I considered adding `Class::isConvertibleFrom` method but
>> we should wait for more use cases proving the need of such convenience method.  This check can be done by calling
>> `Class::referenceType` and `Class::valueType`.
>
> src/java.base/share/classes/java/lang/invoke/AbstractValidatingLambdaMetafactory.java line 398:
> 
>> 397:      */
>> 398:     private boolean canConvert(Class<?> fromType, Class<?> toType) {
>> 399:         if (toType.isAssignableFrom(fromType)) {
> 
> Can this be a static method?

Yes.  OTOH, this is consistent with the existing convention with other similar methods such as `isAdaptableTo` which
are not static.

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PR: https://git.openjdk.java.net/valhalla/pull/102