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  1. JDK
  2. JDK-8174983

LambdaMetafactory: standardize supported type conversions

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      The conversions allowed by LambdaMetafactory on parameter and return types are ad hoc and inconsistent with the specification.

      Propose standardizing on the conversions supported by MethodHandle.asType, which are similar but a little more flexible.

      Specifically:

      factoryType.parameterType(i) --> implementation.type().parameterType(i)
      Currently: same type (but receiver can be a subtype)
      Change: allow any 'asType' conversion, including boxing & unboxing

      dynamicMethodType.parameterType(i) --> implementation.type().parameterType(j)
      Currently: ref subtype, prim widening, or box/unbox followed by these (but receiver can't be boxed)
      Change: allow conversion between arbitrary refs; allow narrowing from a supertype before unbox; allow receiver to be boxed

      interfaceMethodType.parameterType(i) --> dynamicMethodType.parameterType(i)
      Currently: ref supertype, prim same
      Change: allow any 'asType' conversion, including boxing & unboxing

      implementation.type().returnType() --> dynamicMethodType.returnType()
      Currently: any type targeting void, arbitrary refs, prim widening, box+widening, unbox+widening*
      Change: allow void to target a non-void type; allow narrowing from a supertype before unbox

      dynamicMethodType.returnType() --> interfaceMethodType.returnType()
      Currently: void->void, ref subtype, prim widening, box+widening, unbox+widening
      Change: allow either to be void; allow conversion between arbitrary refs; allow narrowing from a supertype before unbox

      (*Current implementation allows any reference-typed implementation return to be compatible with any expected primitive type, as long as the reference type is not a box type (if it is a box type, unbox+widening applies). This is probably a bug.)

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              dlsmith Dan Smith
              dlsmith Dan Smith
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                Updated: