fix(arrow/compute): support casting between extension types - #1052
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Casts are dispatched on the target type, and the cast function registered for extension targets only accepted NULL inputs. Casting an extension array to a different extension type therefore failed with "function 'cast_extension' has no kernel matching input types" even when the cast between the two storage types is well defined, while the reverse direction (extension input, concrete target) has always been handled by the generic CastFromExtension unwrap kernel. Register a generic extension -> extension kernel that unwraps the input array, casts its storage to the target's storage type honoring the caller's cast options, and rewraps the result as the target extension type. Re-typing between extension types that share a storage type is zero-copy since the inner storage cast is then a no-op. Also correct the GetCommonCastKernels doc comment, which claimed to return kernels for casting from extension types; it only ever returned the cast-from-null kernel.
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Casts are dispatched on the target type, and the cast function registered for extension targets only accepted NULL inputs. Casting an extension array to a different extension type therefore failed with "function 'cast_extension' has no kernel matching input types" even when the cast between the two storage types is well defined, while the reverse direction (extension input, concrete target) has always been handled by the generic
CastFromExtensionunwrap kernel.Register a generic extension -> extension kernel that unwraps the input array, casts its storage to the target's storage type honoring the caller's cast options, and rewraps the result as the target extension type. Re-typing between extension types that share a storage type is zero-copy since the inner storage cast is then a no-op.
Also correct the
GetCommonCastKernelsdoc comment, which claimed to return kernels for casting from extension types; it only ever returned the cast-from-null kernel.