Fix undefined slice error

hypothesis-python/RELEASE.rst

@@ -1,3 +1,6 @@
 RELEASE_TYPE: minor
 
 This release allows :func:`~hypothesis.strategies.slices` to generate ``step=None``,
+and fixes an off-by-one error where the ``start`` index could be equal to ``size``.
+This works fine for all Python sequences and Numpy arrays, but is undefined behaviour
+in the `Array API standard <https://data-apis.org/>`__ (see :pull:`3065`).

hypothesis-python/src/hypothesis/strategies/_internal/core.py

@@ -1901,12 +1901,9 @@ def slices(draw: Any, size: int) -> slice:
     if size == 0:
         step = draw(none() | integers().filter(bool))
         return slice(None, None, step)
-
-    min_start = min_stop = 0
-    max_start = max_stop = size
     # For slices start is inclusive and stop is exclusive
-    start = draw(integers(min_start, max_start) | none())
-    stop = draw(integers(min_stop, max_stop) | none())
+    start = draw(integers(0, size - 1) | none())
+    stop = draw(integers(0, size) | none())
 
     # Limit step size to be reasonable
     if start is None and stop is None:

hypothesis-python/tests/cover/test_slices.py

@@ -33,8 +33,8 @@ def test_stop_stays_within_bounds(size):
 @use_several_sizes
 def test_start_stay_within_bounds(size):
     assert_all_examples(
-        st.slices(size),
-        lambda x: x.start is None or (x.start >= -size and x.start <= size),
+        st.slices(size).filter(lambda x: x.start is not None),
+        lambda x: range(size)[x.start] or True,  # no IndexError raised
     )