Please allow compile-time evaluation of DataType::id() (constexpr), platform-independently

[N-Dekker]

DataType::id() is defined "constexpr", at

tensorstore/tensorstore/data_type.h

Line 696 in d5edeac

constexpr DataTypeId id() const { return operations_->id; }

However, it appears that it can only be evaluated at compile-time when using MSVC, because it depends on TENSORSTORE_DATA_TYPE_CONSTEXPR_OPERATIONS, which is only defined for MSVC:

tensorstore/tensorstore/data_type.h

Lines 86 to 92 in d5edeac

	#ifdef _MSC_VER
	// On MSVC, if `MakeDataTypeOperations<T>::operations` is not declared
	// constexpr, it is initialized dynamically, which can happen too late if
	// `DataType` is used from a global dynamic initializer, e.g. in order to
	// allocate an Array.
	#define TENSORSTORE_DATA_TYPE_CONSTEXPR_OPERATIONS
	#endif

TENSORSTORE_DATA_TYPE_CONSTEXPR_OPERATIONS specifies whether or not MakeDataTypeOperations::operations is constexpr:

tensorstore/tensorstore/data_type.h

Lines 1145 to 1149 in d5edeac

	#ifdef TENSORSTORE_DATA_TYPE_CONSTEXPR_OPERATIONS
	static constexpr internal::DataTypeOperations operations =
	DataTypeOperationsImpl<T>;
	#else
	static const internal::DataTypeOperations operations;

As a consequence, a client function that calls DataType::id() may only evaluated at compile-time evaluation when using MSVC. This appears inconvenient to me.

• I encountered this issue when preparing the following pull request: Evaluate tensorstoreToITKComponentType at compile-time, using constexpr  InsightSoftwareConsortium/ITKIOOMEZarrNGFF#68 It appears to work well on Windows (MSVC), but did not compile on Ubuntu (GCC).

Would it be possible to allow compile-time evaluation of DataType::id() on any of the supported platforms?

[N-Dekker]

Thank you very much, @laramiel