Series.astype(np.integer) doesn't show numpy warning

pandas/core/dtypes/common.py

@@ -8,6 +8,7 @@
     Any,
     Callable,
 )
+import warnings
 
 import numpy as np
 
@@ -1694,7 +1695,12 @@ def pandas_dtype(dtype) -> DtypeObj:
     # try a numpy dtype
     # raise a consistent TypeError if failed
     try:
-        npdtype = np.dtype(dtype)
+        with warnings.catch_warnings():
+            # GH#51523 - Series.astype(np.integer) doesn't show
+            # numpy deprication warning of np.integer
+            # Hence enabling DeprecationWarning
+            warnings.simplefilter("always", DeprecationWarning)
+            npdtype = np.dtype(dtype)
     except SyntaxError as err:
         # np.dtype uses `eval` which can raise SyntaxError
         raise TypeError(f"data type '{dtype}' not understood") from err

pandas/tests/dtypes/test_common.py

@@ -1,5 +1,7 @@
 from __future__ import annotations
 
+import warnings
+
 import numpy as np
 import pytest
 
@@ -754,3 +756,12 @@ def test_validate_allhashable():
 
     with pytest.raises(TypeError, match="list must be a hashable type"):
         com.validate_all_hashable([], error_name="list")
+
+
+def test_pandas_dtype_numpy_warning():
+    # GH#51523
+    with warnings.catch_warnings(record=True) as w:
+        warnings.simplefilter("always")
+        pandas_dtype(np.integer)
+        assert len(w) == 1  # should have raised one warning
+        assert "DeprecationWarning" in str(w[-1])  # we get the default message