CI Adapt handling of discarded fused typed memoryview (#25425)

Co-authored-by: Olivier Grisel <olivier.grisel@ensta.org>

sklearn/datasets/_svmlight_format_fast.pyx

@@ -187,6 +187,7 @@ def _dump_svmlight_file(
     bint y_is_sp,
 ):
     cdef bint X_is_integral
+    cdef bint query_id_is_not_empty = query_id.size > 0
     X_is_integral = X.dtype.kind == "i"
     if X_is_integral:
         value_pattern = "%d:%d"
@@ -198,7 +199,7 @@ def _dump_svmlight_file(
         label_pattern = "%.16g"
 
     line_pattern = "%s"
-    if query_id is not None:
+    if query_id_is_not_empty:
         line_pattern += " qid:%d"
     line_pattern += " %s\n"
 
@@ -246,7 +247,7 @@ def _dump_svmlight_file(
             else:
                 labels_str = label_pattern % y[i,0]
 
-        if query_id is not None:
+        if query_id_is_not_empty:
             feat = (labels_str, query_id[i], s)
         else:
             feat = (labels_str, s)

sklearn/datasets/_svmlight_format_io.py

@@ -506,7 +506,13 @@ def dump_svmlight_file(
         if hasattr(X, "sort_indices"):
             X.sort_indices()
 
-    if query_id is not None:
+    if query_id is None:
+        # NOTE: query_id is passed to Cython functions using a fused type on query_id.
+        # Yet as of Cython>=3.0, memory views can't be None otherwise the runtime
+        # would not known which concrete implementation to dispatch the Python call to.
+        # TODO: simplify interfaces and implementations in _svmlight_format_fast.pyx.
+        query_id = np.array([], dtype=np.int32)
+    else:
         query_id = np.asarray(query_id)
         if query_id.shape[0] != y.shape[0]:
             raise ValueError(