DOC: add warning about typing-extensions module to numpy.typing docs

Typing `ArrayLike` correctly relies on `Protocol`, so warn users that
they should be on 3.8+ or install `typing-extensions` if they want
everything to work as expected.

2 files changed, 11 insertions, 2 deletions

diff --git a/numpy/typing/__init__.py b/numpy/typing/__init__.py
index bc3442a15..1d377f8c4 100644
--- a/numpy/typing/__init__.py
+++ b/numpy/typing/__init__.py
@@ -3,6 +3,13 @@
 Typing (:mod:`numpy.typing`)
 ============================
 
+.. warning::
+
+  Some of the types in this module rely on features only present in
+  the standard library in Python 3.8 and greater. If you want to use
+  these types in earlier versions of Python, you should install the
+  typing-extensions_ package.
+
 Large parts of the NumPy API have PEP-484-style type annotations. In
 addition, the following type aliases are available for users.
 
@@ -13,6 +20,8 @@ Roughly speaking, ``typing.ArrayLike`` is "objects that can be used as
 inputs to ``np.array``" and ``typing.DtypeLike`` is "objects that can
 be used as inputs to ``np.dtype``".
 
+.. _typing-extensions: https://pypi.org/project/typing-extensions/
+
 Differences from the runtime NumPy API
 --------------------------------------
 
diff --git a/numpy/typing/_array_like.py b/numpy/typing/_array_like.py
index 54a612fb4..b73585cf0 100644
--- a/numpy/typing/_array_like.py
+++ b/numpy/typing/_array_like.py
@@ -1,5 +1,5 @@
 import sys
-from typing import Any, overload, Sequence, Tuple, Union
+from typing import Any, overload, Sequence, TYPE_CHECKING, Union
 
 from numpy import ndarray
 from ._dtype_like import DtypeLike
@@ -15,7 +15,7 @@ else:
     else:
         HAVE_PROTOCOL = True
 
-if HAVE_PROTOCOL:
+if TYPE_CHECKING or HAVE_PROTOCOL:
     class _SupportsArray(Protocol):
         @overload
         def __array__(self, __dtype: DtypeLike = ...) -> ndarray: ...