REGR: read_csv and nullable dtypes fails if dtype is specified (#49147)

Author: phofl

pandas/_libs/parsers.pyx

@@ -1414,6 +1414,9 @@ def _maybe_upcast(arr, use_nullable_dtypes: bool = False):
     -------
     The casted array.
     """
+    if is_extension_array_dtype(arr.dtype):
+        return arr
+
     na_value = na_values[arr.dtype]
 
     if issubclass(arr.dtype.type, np.integer):

pandas/tests/io/parser/dtypes/test_dtypes_basic.py

@@ -466,3 +466,14 @@ def test_use_nullabla_dtypes_string(all_parsers, storage):
                 }
             )
         tm.assert_frame_equal(result, expected)
+
+
+def test_use_nullable_dtypes_ea_dtype_specified(all_parsers):
+    # GH#491496
+    data = """a,b
+1,2
+"""
+    parser = all_parsers
+    result = parser.read_csv(StringIO(data), dtype="Int64", use_nullable_dtypes=True)
+    expected = DataFrame({"a": [1], "b": 2}, dtype="Int64")
+    tm.assert_frame_equal(result, expected)