ENH: Expose ufunc.resolve_dtypes and strided loop access

The API here designed is "future" in the sense that it implementes
NEP 50 and exposes loop specializations as per NEP 43 which is barely
used by NumPy itself at this point.

Due to the fact that NEP 50 is not implemented (or rather finalized)
this API is not ideal and creates dummy-arrays internally so that
it is not much faster than if the user created dummy arrays to
probe the correct result.

numpy/core/_add_newdocs.py

@@ -5694,6 +5694,133 @@
 
     """))
 
+add_newdoc('numpy.core', 'ufunc', ('resolve_dtypes',
+    """
+    resolve_dtypes(dtypes, *, signature=None, casting=None, reduction=False)
+
+    Find the dtypes NumPy will use for the operation.  Both input and
+    output dtypes are returned and may differ from those provided.
+
+    .. note::
+
+        This function always applies NEP 50 rules since it is not provided
+        and actual values.  The Python types ``int``, ``float``, and
+        ``complex`` thus behave weak and should be passed for "untyped"
+        Python input.
+
+    Parameters
+    ----------
+    dtypes : tuple of dtypes, None, or literal int, float, complex
+        The input dtypes for each operand.  Output operands can be left
+        None, indicating that the dtype must be found.
+    signature : tuple of DTypes or None, optional
+        If given, enforces exact DType (classes) of the specific operand.
+        The ufunc ``dtype`` argument is equivalent to passing a tuple with
+        only output dtypes set.
+    casting : {'no', 'equiv', 'safe', 'same_kind', 'unsafe'}, optional
+        The casting mode when casting is necessary.  This is identical to
+        the ufunc call casting modes.
+    reduction : boolean
+        If given, the resolution assumes a reduce operation is happening
+        which slightly changes the promotion and type resolution rules.
+
+    Returns
+    -------
+    dtypes : tuple of dtypes
+        The dtypes which NumPy would use for the calculation.  Note that
+        dtypes may not match the passed in ones (casting is necessary).
+
+    See Also
+    --------
+    numpy.ufunc._resolve_dtypes_and_context :
+        Similar function to this, but returns additional information which
+        give access to the core C functionality of NumPy.
+
+    """))
+
+add_newdoc('numpy.core', 'ufunc', ('_resolve_dtypes_and_context',
+    """
+    _resolve_dtypes_and_context(dtypes, *, signature=None, casting=None, reduction=False)
+
+    See `numpy.ufunc.resolve_dtypes` for parameter information.  This
+    function is considered *unstable*.  You may use it, but the returned
+    information is NumPy version specific and expected to change.
+    Large API/ABI changes are not expected, but a new NumPy version is
+    expected to require updating code using this functionality.
+
+    This function is designed to be used in conjunction with
+    `numpy.ufunc._get_strided_loop`.  The calls are split to mirror the C API
+    and allow future improvements.
+
+    Returns
+    -------
+    dtypes : tuple of dtypes
+    call_info :
+        PyCapsule with all necessary information to get access to low level
+        C calls.  See `numpy.ufunc._get_strided_loop` for more information.
+
+    """))
+
+add_newdoc('numpy.core', 'ufunc', ('_get_strided_loop',
+    """
+    _get_strided_loop(call_info, /, *, fixed_strides=None)
+
+    This function fills in the ``call_info`` capsule to include all
+    information necessary to call the low-level strided loop from NumPy.
+
+    See notes for more information.
+
+    Parameters
+    ----------
+    call_info : PyCapsule
+        The PyCapsule returned by `numpy.ufunc._resolve_dtypes_and_context`.
+    fixed_strides : tuple of int or None, optional
+        A tuple with fixed byte strides of all input arrays.  NumPy may use
+        this information to find specialized loops, so any call must follow
+        the given stride.  Use ``None`` to indicate that the stride is not
+        known (or not fixed) for all calls.
+
+    Notes
+    -----
+    Together with `numpy.ufunc._resolve_dtypes_and_context` this function
+    gives low-level access to the NumPy ufunc loops.
+    The first function does general preparations and returns the required
+    information. It returns this as a C capsule with the version specific
+    name ``numpy_1.24_ufunc_call_info``.
+    The NumPy 1.24 ufunc call info capsule has the following layout::
+
+        typedef struct {
+            PyArrayMethod_StridedLoop *strided_loop;
+            PyArrayMethod_Context context;
+            NpyAuxData *auxdata;
+
+            /* Flag information (expected to change) */
+            npy_bool requires_pyapi;  /* GIL is required by loop */
+
+            /* Loop doesn't set FPE flags; if not set check FPE flags */
+            npy_bool no_floatingpoint_errors;
+        } ufunc_call_info;
+
+    Note that the first call only fills in the ``context``.  The call to
+    ``_get_strided_loop`` fills in all other data.
+    Please see the ``numpy/experimental_dtype_api.h`` header for exact
+    call information; the main thing to note is that the new-style loops
+    return 0 on success, -1 on failure.  They are passed context as new
+    first input and ``auxdata`` as (replaced) last.
+
+    Only the ``strided_loop``signature is considered guaranteed stable
+    for NumPy bug-fix releases.  All other API is tied to the experimental
+    API versioning.
+
+    The reason for the split call is that cast information is required to
+    decide what the fixed-strides will be.
+
+    NumPy ties the lifetime of the ``auxdata`` information to the capsule.
+
+    """))
+
+
+
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 #
 # Documentation for dtype attributes and methods