[C++] const qualifier on mutable_span_as prevents call to mutable_data_as without const qualifier

[sjperkins]

Describe the bug, including details regarding any error messages, version, and platform.

#38027 added Buffer::span_as and Buffer::mutable_span_as

mutable_span_as has a const qualifier,

arrow/cpp/src/arrow/buffer.h

Lines 265 to 274 in 53e0c74

	template <typename T>
	T* mutable_data_as() {
	return reinterpret_cast<T*>(mutable_data());
	}
	/// \brief Return the buffer's mutable data as a span
	template <typename T>
	util::span<T> mutable_span_as() const {
	return util::span(mutable_data_as<T>(), static_cast<size_t>(size() / sizeof(T)));
	}

but this prevents the call to mutable_data_as during compile:

...include/arrow/buffer.h:273:41: error: passing ‘const arrow::Buffer’ as ‘this’ argument discards qualifiers [-fpermissive]
  273 |     return util::span(mutable_data_as<T>(), static_cast<size_t>(size() / sizeof(T)));

By contrast, both span_as and data_as have const qualifiers.

arrow/cpp/src/arrow/buffer.h

Lines 228 to 241 in 53e0c74

	/// \brief Return a pointer to the buffer's data cast to a specific type
	///
	/// The buffer has to be a CPU buffer (`is_cpu()` is true).
	/// Otherwise, an assertion may be thrown or a null pointer may be returned.
	template <typename T>
	const T* data_as() const {
	return reinterpret_cast<const T*>(data());
	}
	/// \brief Return the buffer's data as a span
	template <typename T>
	util::span<const T> span_as() const {
	return util::span(data_as<T>(), static_cast<size_t>(size() / sizeof(T)));
	}

Also, mutable_span_as should not have a const qualifier as it implies modification of arrow::Buffer.

Component(s)

C++

[kou]

Issue resolved by pull request 40367
#40367