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LW2 is an iteration of a previous prototype, adding further language support and JDK API support for "inline - types" (formerly referred to as "value types"). There are also subtle type system, runtime and JIT changes

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• Requires source level >= JDK13 (since LW2, the prototype is based on mainline JDK, whose version string is JDK 14 at the time of writing)
• A class declaration of inline type is made by using the "inline class" modifier, or (for IDE syntax parsing convenience) with "@__inline__" annotation (since LW2)
  
  • Interfaces, annotation types, enums can not be inline types
  • Top level, inner, nested, local classes may be inline types. Inline types may declare inner, nested, local types
  • Inline Types are implicitly final, so cannot be abstract
• Inline Types may not declare an explicit super class (except Object). They implicitly extend java.lang.Object akin to enums, annotation types and interfaces
• Inline Types may declare explicit interfaces
• Inline Types constructors may not pass the "this" handle until all instance fields are definitely assigned.
• All instance fields of a inline class are implicitly finalInline types may not to declare fields of its own type directly.
  There is a method to do this "indirectly" using the "?" operator, but it's value is still final. (since LW2)
• "Indirect" projections of inline types via the "?" operator (since LW2)
  • E.g. the projection of Point is Point?
  • Point? IS also an inline type which allows null for backward compatibility
  • maintains type of a field,  array element, or generic parametric argument, but they may be null
  • Point is a subtype of Point?
  • cast between inline type and indirect type
  • Is implicitly a "NullableType"
  • enables compatibility with current generics via erasure to null-object object references
  • Arrays subtyping relationships: InlineType[] <: IndirectType[] <: Object[] <: Object
    • Widening conversions may be carried out from in the order above
    • Narrowing from an array of IndirectType to an InlineType is not possible.
    • This array covariance enables compatibility with existing API, especially generic type erasure
• Inline types may not declare fields of its own type directly.  If A and B are inline classes, A may not contain a field B if B contains a field A.
  
  • There is a method to do this "indirectly" using the "?" operator, but it's value is still final. (since LW2
• java.lang.Object methods:
  • javac automatically generates hashCode, equals, longHashCode and toString computed solely from the instance's state and not from its identity
  • javac does not clone(), finalize(), wait*), notify*() on inline type receivers
• javac allows comparison of inline type using ==, != (since LW2)
  • By default this implies a test in the instance state (i.e. field by field "substitutability" test), as inline type types have no identity (i.e. not a reference test).
• Inline Types can not be assigned null, null can not be cast to or compared with inline types
  • With the exception if of an "indirect" reference using the "?" operator. (since LW2)
• Indirect Types cannot be type arguments in generic type parameterizations, type witnesses in generic method invocations, wildcard bounds
  • With the exception if of an "indirect" reference using the "?" operator. (since LW2) 
• Type migration, including partial recompilation is not supported.

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• Class new /modified API:
  • isInlineClass()
  • asPrimaryType()
  • asIndirectType() / isIndirectType()
  • asNullableType() / isNullableType()
  • getName() reflects the Q or L type signatures for arrays of inline types (since LW2)
    • In the .class file, the null-free inline type today is represented by a Q type signature, and the indirect projection is represented by an L type descriptor
  • newInstance() on a an inline type throws NoSuchMethodException
• setAccessible() on a inline type throws InaccessibleObjectException
• Initial core Reflection and VarHandles support in place (since LW2)
• Attempts to build Reference objects with inline type will result in IllegalArgumentException

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• IllegalMonitorException thrown on attempts to synchronize or call wait(*) or notify*() on a an inline type
• ClassCircularityError thrown if loading an instance field of a inline type which declares its own type either directly
• Attempts to serialize an inline type will throw NotSerializableException
• Cast from indirect type to inline type may result in NullPointerException (since LW2)
• Attempts to store "null" into Inline type array will throw NullPointerException
  • Where as Whereas null into indirect type array works as normal (since LW2)

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• Use cases that worked for you, including any positive performance or footprint information
• Bugs or questions about surprise behavior
• We have performance holes, if you have a problem, please provide a use case for us to consider

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