note that passing axes with repeated entries is UB

Author: leofang

src/array_api_stubs/_draft/fft.py

@@ -146,6 +146,8 @@ def fftn(
         axes (dimensions) over which to compute the transform. A valid axis in ``axes`` must be an integer on the interval ``[-N, N)``, where ``N`` is the rank (number of dimensions) of ``x`` along that axis. If an axis is specified as a negative integer, the function must determine the axis along which to compute the transform by counting backward from the last dimension (where ``-1`` refers to the last dimension).
 
         If ``s`` is provided, the corresponding ``axes`` to be transformed must also be provided. If ``axes`` is ``None``, the function must compute the transform over all axes. Default: ``None``.
+
+        If ``axes`` contains any repeated entries, the behavior is unspecified and thus implementation-defined.
     norm: Literal['backward', 'ortho', 'forward']
         normalization mode. Should be one of the following modes:
 
@@ -199,6 +201,8 @@ def ifftn(
         axes (dimensions) over which to compute the transform. A valid axis in ``axes`` must be an integer on the interval ``[-N, N)``, where ``N`` is the rank (number of dimensions) of ``x`` along that axis. If an axis is specified as a negative integer, the function must determine the axis along which to compute the transform by counting backward from the last dimension (where ``-1`` refers to the last dimension).
 
         If ``s`` is provided, the corresponding ``axes`` to be transformed must also be provided. If ``axes`` is ``None``, the function must compute the transform over all axes. Default: ``None``.
+
+        If ``axes`` contains any repeated entries, the behavior is unspecified and thus implementation-defined.
     norm: Literal['backward', 'ortho', 'forward']
         specify the normalization mode. Should be one of the following modes:
 
@@ -352,6 +356,8 @@ def rfftn(
         axes (dimensions) over which to compute the transform. A valid axis in ``axes`` must be an integer on the interval ``[-N, N)``, where ``N`` is the rank (number of dimensions) of ``x`` along that axis. If an axis is specified as a negative integer, the function must determine the axis along which to compute the transform by counting backward from the last dimension (where ``-1`` refers to the last dimension).
 
         If ``s`` is provided, the corresponding ``axes`` to be transformed must also be provided. If ``axes`` is ``None``, the function must compute the transform over all axes. Default: ``None``.
+
+        If ``axes`` contains any repeated entries, the behavior is unspecified and thus implementation-defined.
     norm: Literal['backward', 'ortho', 'forward']
         normalization mode. Should be one of the following modes:
 
@@ -404,7 +410,9 @@ def irfftn(
     axes: Optional[Sequence[int]]
         axes (dimensions) over which to compute the transform. A valid axis in ``axes`` must be an integer on the interval ``[-N, N)``, where ``N`` is the rank (number of dimensions) of ``x`` along that axis. If an axis is specified as a negative integer, the function must determine the axis along which to compute the transform by counting backward from the last dimension (where ``-1`` refers to the last dimension).
 
-       If ``s`` is provided, the corresponding ``axes`` to be transformed must also be provided. If ``axes`` is ``None``, the function must compute the transform over all axes. Default: ``None``.
+        If ``s`` is provided, the corresponding ``axes`` to be transformed must also be provided. If ``axes`` is ``None``, the function must compute the transform over all axes. Default: ``None``.
+
+        If ``axes`` contains any repeated entries, the behavior is unspecified and thus implementation-defined.
     norm: Literal['backward', 'ortho', 'forward']
         normalization mode. Should be one of the following modes:
 
@@ -604,6 +612,8 @@ def fftshift(x: array, /, *, axes: Optional[Union[int, Sequence[int]]] = None) -
     axes: Optional[Union[int, Sequence[int]]]
         axes over which to shift. If ``None``, the function must shift all axes. Default: ``None``.
 
+        If ``axes`` contains any repeated entries, the behavior is unspecified and thus implementation-defined.
+
     Returns
     -------
     out: array
@@ -632,6 +642,8 @@ def ifftshift(
     axes: Optional[Union[int, Sequence[int]]]
         axes over which to perform the inverse shift. If ``None``, the function must shift all axes. Default: ``None``.
 
+        If ``axes`` contains any repeated entries, the behavior is unspecified and thus implementation-defined.
+
     Returns
     -------
     out: array