RFR: 4517644: Core reflection setter type conversion spec bugs [v2]

[Chen Liang]

> ### Current Status
> In core reflection, there are a few holes with the documentation on the type conversions performed on the different setters. They generally follow this process:
> 
> 1. The Object-valued setters checks the destination type. If that type is primitive, unbox the object.
> 2. Perform identity or (primitive/reference, depending on destination type) widening conversions.
> 
> For each step, they can fail with IllegalArgumentException, which has been the long standing behavior.
> 
> Note that this process is more restrictive than the JLS 5.2 assignment context, which allows boxing conversions, while the primitive reflective setters consistently reject them.
> 
> ### Problems
> There are some problems with current specs:
> 1. No mention that boxing is never done for primitive-typed setters
> 2. Array.set missing description for the final identity or widening conversion (reference or primitive) and the associated IAE condition
> 3. Class specification of field mentions widening/narrowing conversion rules that is not sensible for get/set Object accessors
> 4. Field.set misses the identity or widening in IAE clause (but mentioned in main body)
> 5. Field primitive setters incorrectly claim they are equivalent to set(instance, wrapper) which is wrong due to lack of boxing conversion
> 6. Field primitive setters refer to nonsense "unwrapping conversion"
> 
> ### Solutions
> 1. Make sure the unbox -> identity/widen process and the IAE conditions are present in both Field and Array.set
> 2. Add that boxing is absent for all primitive setters
> 3. Update Field class spec to mention the narrowing/widening conversion limits are for primitive accessors only
> 4. Fix the Field primitive setters' "unwrapping conversion" to be "identity or primitive widening conversion" as in Array primitive setters
> 5. Qualify the Field primitive setters assertion with "if this field is of a primitive type" to make it correct

Chen Liang has updated the pull request incrementally with one additional commit since the last revision:

  the narrowing conversion restriction is primitive-only

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/28029/files
  - new: https://git.openjdk.org/jdk/pull/28029/files/9f925617..5559e613

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=28029&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=28029&range=00-01

  Stats: 3 lines in 1 file changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/28029.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28029/head:pull/28029

PR: https://git.openjdk.org/jdk/pull/28029