[C++] Support casting from storage type to extension type

[asfimport]

We already support casting an extension array to its storage type (ARROW-7858):

>>> from pyarrow.tests.test_extension_type import IntegerType
>>> arr = pa.array([1, 2, 3], pa.int64())
>>> arr_ext = pa.ExtensionArray.from_storage(IntegerType(), arr)
>>> arr_ext.cast(arr.type)
Out[29]: 
<pyarrow.lib.Int64Array object at 0x7f7b0d4739a0>
[
 1,
 2,
 3
]

but we don't support the cast the other way around from storage type to extension type:

>>> arr.cast(arr_ext.type)
...
ArrowNotImplementedError: Unsupported cast from int64 to extension<arrow.py_extension_type<IntegerType>> (no available cast function for target type)
../src/arrow/compute/cast.cc:119 GetCastFunctionInternal(cast_options->to_type, args[0].type().get())

Reporter: Joris Van den Bossche / @jorisvandenbossche
Assignee: Miles Granger / @milesgranger

Related issues:

• [C++][Python] Support casting an Extension type to its storage type (is related to)
• [C++] Cast dictionary of extension type to extension type (is related to)

PRs and other links:

• GitHub Pull Request #14106

_{Note: This issue was originally created as ARROW-14500. Please see the migration documentation for further details.}

[asfimport]

Will Ayd / @WillAyd:
Did a little research here. I see the casting from an extension type is handled here:

arrow/cpp/src/arrow/compute/kernels/scalar_cast_internal.cc

Line 190 in e64480d

Status CastFromExtension(KernelContext* ctx, const ExecBatch& batch, Datum* out) {

So the other way around could be pretty similar. However, I'm wondering if a cast to an extension type is truly possible? When coming from an extension type the built-in type can be cast from the extension's physical storage. But would going the other way around would require a creation of a new ExtensionArray instance that populates some of the additional methods?

[asfimport]

Joris Van den Bossche / @jorisvandenbossche:

require a creation of a new ExtensionArray instance that populates some of the additional methods?

The ExtensionType instance can create such an array, see eg

arrow/cpp/src/arrow/extension_type.h

Lines 87 to 89 in e64480d

	/// \brief Wrap the given storage array as an extension array
	static std::shared_ptr<Array> WrapArray(const std::shared_ptr<DataType>& ext_type,
	const std::shared_ptr<Array>& storage);

(ExtensionType::MakeArray is a required method for extension authors to implement)

[asfimport]

Joris Van den Bossche / @jorisvandenbossche:
This is being addressed in #14106

[asfimport]

Joris Van den Bossche / @jorisvandenbossche:
Resolved by #14106