MAINT: Move dtype instance to DType class cast

This function is better housed in convert_datatype.c and was only
in array_coercion, because we did not use it anywhere else before.

This also somewhat modifies the logic and cleans up use-cases of
it in array_coercion.c

3 files changed, 62 insertions, 51 deletions

diff --git a/numpy/core/src/multiarray/array_coercion.c b/numpy/core/src/multiarray/array_coercion.c
index ffb5bd632..4e2354991 100644
--- a/numpy/core/src/multiarray/array_coercion.c
+++ b/numpy/core/src/multiarray/array_coercion.c
@@ -306,51 +306,6 @@ discover_dtype_from_pyobject(
 }
 
 
-/*
- * This function should probably become public API eventually.  At this
- * time it is implemented by falling back to `PyArray_AdaptFlexibleDType`.
- * We will use `CastingImpl[from, to].adjust_descriptors(...)` to implement
- * this logic.
- */
-static NPY_INLINE PyArray_Descr *
-cast_descriptor_to_fixed_dtype(
-        PyArray_Descr *descr, PyArray_DTypeMeta *fixed_DType)
-{
-    if (fixed_DType == NULL) {
-        /* Nothing to do, we only need to promote the new dtype */
-        Py_INCREF(descr);
-        return descr;
-    }
-
-    if (!fixed_DType->parametric) {
-        /*
-         * Don't actually do anything, the default is always the result
-         * of any cast.
-         */
-        return fixed_DType->default_descr(fixed_DType);
-    }
-    if (PyObject_TypeCheck((PyObject *)descr, (PyTypeObject *)fixed_DType)) {
-        Py_INCREF(descr);
-        return descr;
-    }
-    /*
-     * TODO: When this is implemented for all dtypes, the special cases
-     *       can be removed...
-     */
-    if (fixed_DType->legacy && fixed_DType->parametric &&
-            NPY_DTYPE(descr)->legacy) {
-        PyArray_Descr *flex_dtype = PyArray_DescrFromType(fixed_DType->type_num);
-        return PyArray_AdaptFlexibleDType(descr, flex_dtype);
-    }
-
-    PyErr_SetString(PyExc_NotImplementedError,
-            "Must use casting to find the correct dtype, this is "
-            "not yet implemented! "
-            "(It should not be possible to hit this code currently!)");
-    return NULL;
-}
-
-
 /**
  * Discover the correct descriptor from a known DType class and scalar.
  * If the fixed DType can discover a dtype instance/descr all is fine,
@@ -392,7 +347,7 @@ find_scalar_descriptor(
         return descr;
     }
 
-    Py_SETREF(descr, cast_descriptor_to_fixed_dtype(descr, fixed_DType));
+    Py_SETREF(descr, PyArray_CastDescrToDType(descr, fixed_DType));
     return descr;
 }
 
@@ -727,8 +682,13 @@ find_descriptor_from_array(
     enum _dtype_discovery_flags flags = 0;
     *out_descr = NULL;
 
-    if (NPY_UNLIKELY(DType != NULL && DType->parametric &&
-            PyArray_ISOBJECT(arr))) {
+    if (DType == NULL) {
+        *out_descr = PyArray_DESCR(arr);
+        Py_INCREF(*out_descr);
+        return 0;
+    }
+
+    if (NPY_UNLIKELY(DType->parametric && PyArray_ISOBJECT(arr))) {
         /*
          * We have one special case, if (and only if) the input array is of
          * object DType and the dtype is not fixed already but parametric.
@@ -777,7 +737,7 @@ find_descriptor_from_array(
         }
         Py_DECREF(iter);
     }
-    else if (DType != NULL && NPY_UNLIKELY(DType->type_num == NPY_DATETIME) &&
+    else if (NPY_UNLIKELY(DType->type_num == NPY_DATETIME) &&
                 PyArray_ISSTRING(arr)) {
         /*
          * TODO: This branch should be deprecated IMO, the workaround is
@@ -806,8 +766,7 @@ find_descriptor_from_array(
          * If this is not an object array figure out the dtype cast,
          * or simply use the returned DType.
          */
-        *out_descr = cast_descriptor_to_fixed_dtype(
-                     PyArray_DESCR(arr), DType);
+        *out_descr = PyArray_CastDescrToDType(PyArray_DESCR(arr), DType);
         if (*out_descr == NULL) {
             return -1;
         }
diff --git a/numpy/core/src/multiarray/convert_datatype.c b/numpy/core/src/multiarray/convert_datatype.c
index d9121707b..0ccc0b538 100644
--- a/numpy/core/src/multiarray/convert_datatype.c
+++ b/numpy/core/src/multiarray/convert_datatype.c
@@ -1031,6 +1031,55 @@ ensure_dtype_nbo(PyArray_Descr *type)
     }
 }
 
+
+/**
+ * This function should possibly become public API eventually.  At this
+ * time it is implemented by falling back to `PyArray_AdaptFlexibleDType`.
+ * We will use `CastingImpl[from, to].adjust_descriptors(...)` to implement
+ * this logic.
+ * Before that, the API needs to be reviewed though.
+ *
+ * WARNING: This function currently does not guarantee that `descr` can
+ *          actually be cast to the given DType.
+ *
+ * @param descr The dtype instance to adapt "cast"
+ * @param given_DType The DType class for which we wish to find an instance able
+ *        to represent `descr`.
+ * @returns Instance of `given_DType`. If `given_DType` is parametric the
+ *          descr may be adapted to hold it.
+ */
+NPY_NO_EXPORT PyArray_Descr *
+PyArray_CastDescrToDType(PyArray_Descr *descr, PyArray_DTypeMeta *given_DType)
+{
+    if (NPY_DTYPE(descr) == given_DType) {
+        Py_INCREF(descr);
+        return descr;
+    }
+    if (!given_DType->parametric) {
+        /*
+         * Don't actually do anything, the default is always the result
+         * of any cast.
+         */
+        return given_DType->default_descr(given_DType);
+    }
+    if (PyObject_TypeCheck((PyObject *)descr, (PyTypeObject *)given_DType)) {
+        Py_INCREF(descr);
+        return descr;
+    }
+
+    if (!given_DType->legacy) {
+        PyErr_SetString(PyExc_NotImplementedError,
+                "Must use casting to find the correct DType for a parametric "
+                "user DType. This is not yet implemented (this error should be "
+                "unreachable).");
+        return NULL;
+    }
+
+    PyArray_Descr *flex_dtype = PyArray_DescrNew(given_DType->singleton);
+    return PyArray_AdaptFlexibleDType(descr, flex_dtype);
+}
+
+
 /*NUMPY_API
  * Produces the smallest size and lowest kind type to which both
  * input types can be cast.
diff --git a/numpy/core/src/multiarray/convert_datatype.h b/numpy/core/src/multiarray/convert_datatype.h
index 9b7f39db2..eef2f7313 100644
--- a/numpy/core/src/multiarray/convert_datatype.h
+++ b/numpy/core/src/multiarray/convert_datatype.h
@@ -49,4 +49,7 @@ npy_set_invalid_cast_error(
 NPY_NO_EXPORT PyArray_Descr *
 PyArray_AdaptFlexibleDType(PyArray_Descr *data_dtype, PyArray_Descr *flex_dtype);
 
+NPY_NO_EXPORT PyArray_Descr *
+PyArray_CastDescrToDType(PyArray_Descr *descr, PyArray_DTypeMeta *given_DType);
+
 #endif