From 3c7261db44a6961c601db3ea92770ba93aa76fa6 Mon Sep 17 00:00:00 2001
From: Patrick Hoefler <61934744+phofl@users.noreply.github.com>
Date: Fri, 25 Aug 2023 16:28:59 +0200
Subject: [PATCH 1/2] BUG: merge raising for ea int and numpy float

---
 doc/source/whatsnew/v2.1.0.rst           |  1 +
 pandas/core/reshape/merge.py             | 10 ++++++++++
 pandas/tests/reshape/merge/test_merge.py | 23 +++++++++++++++++++++++
 3 files changed, 34 insertions(+)

diff --git a/doc/source/whatsnew/v2.1.0.rst b/doc/source/whatsnew/v2.1.0.rst
index fabe910261c3d..cf1cfc9f0aa42 100644
--- a/doc/source/whatsnew/v2.1.0.rst
+++ b/doc/source/whatsnew/v2.1.0.rst
@@ -815,6 +815,7 @@ Reshaping
 - Bug in :func:`merge_asof` raising ``KeyError`` for extension dtypes (:issue:`52904`)
 - Bug in :func:`merge_asof` raising ``ValueError`` for data backed by read-only ndarrays (:issue:`53513`)
 - Bug in :func:`merge_asof` with ``left_index=True`` or ``right_index=True`` with mismatched index dtypes giving incorrect results in some cases instead of raising ``MergeError`` (:issue:`53870`)
+- Bug in :func:`merge` when merging on integer ``ExtensionDtype`` and float NumPy dtype raising ``TypeError`` (:issue:`46178`)
 - Bug in :meth:`DataFrame.agg` and :meth:`Series.agg` on non-unique columns would return incorrect type when dist-like argument passed in (:issue:`51099`)
 - Bug in :meth:`DataFrame.combine_first` ignoring other's columns if ``other`` is empty (:issue:`53792`)
 - Bug in :meth:`DataFrame.idxmin` and :meth:`DataFrame.idxmax`, where the axis dtype would be lost for empty frames (:issue:`53265`)
diff --git a/pandas/core/reshape/merge.py b/pandas/core/reshape/merge.py
index 140a3024a8684..a936903ff3ffa 100644
--- a/pandas/core/reshape/merge.py
+++ b/pandas/core/reshape/merge.py
@@ -53,6 +53,7 @@
     ensure_object,
     is_bool,
     is_bool_dtype,
+    is_extension_array_dtype,
     is_float_dtype,
     is_integer,
     is_integer_dtype,
@@ -1385,6 +1386,15 @@ def _maybe_coerce_merge_keys(self) -> None:
                 if lk.dtype.kind == rk.dtype.kind:
                     continue
 
+                if is_extension_array_dtype(lk.dtype) and not is_extension_array_dtype(
+                    rk.dtype
+                ):
+                    ct = find_common_type([lk.dtype, rk.dtype])
+                    rk = ct.construct_array_type()._from_sequence(rk)
+                elif is_extension_array_dtype(rk.dtype):
+                    ct = find_common_type([lk.dtype, rk.dtype])
+                    lk = ct.construct_array_type()._from_sequence(lk)
+
                 # check whether ints and floats
                 if is_integer_dtype(rk.dtype) and is_float_dtype(lk.dtype):
                     # GH 47391 numpy > 1.24 will raise a RuntimeError for nan -> int
diff --git a/pandas/tests/reshape/merge/test_merge.py b/pandas/tests/reshape/merge/test_merge.py
index 50a534ad36bcc..02d7e2059e8e1 100644
--- a/pandas/tests/reshape/merge/test_merge.py
+++ b/pandas/tests/reshape/merge/test_merge.py
@@ -2924,3 +2924,26 @@ def test_merge_combinations(
         expected = expected.reset_index(drop=True)
 
     tm.assert_frame_equal(result, expected)
+
+
+def test_merge_ea_int_and_float_numpy():
+    # GH#46178
+    df1 = DataFrame([1.0, np.nan], dtype=pd.Int64Dtype())
+    df2 = DataFrame([1.5])
+    expected = DataFrame(columns=[0], dtype="Int64")
+
+    with tm.assert_produces_warning(UserWarning, match="You are merging"):
+        result = df1.merge(df2)
+    tm.assert_frame_equal(result, expected)
+
+    with tm.assert_produces_warning(UserWarning, match="You are merging"):
+        result = df2.merge(df1)
+    tm.assert_frame_equal(result, expected.astype("float64"))
+
+    df2 = DataFrame([1.0])
+    expected = DataFrame([1], columns=[0], dtype="Int64")
+    result = df1.merge(df2)
+    tm.assert_frame_equal(result, expected)
+
+    result = df2.merge(df1)
+    tm.assert_frame_equal(result, expected.astype("float64"))

From cf773a8fc3dc17a045b8e08e004175dafe79254b Mon Sep 17 00:00:00 2001
From: Patrick Hoefler <61934744+phofl@users.noreply.github.com>
Date: Sat, 26 Aug 2023 00:51:23 +0200
Subject: [PATCH 2/2] Fix up mypy and add check

---
 pandas/core/reshape/merge.py | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/pandas/core/reshape/merge.py b/pandas/core/reshape/merge.py
index a936903ff3ffa..13bc1008698b2 100644
--- a/pandas/core/reshape/merge.py
+++ b/pandas/core/reshape/merge.py
@@ -1390,10 +1390,16 @@ def _maybe_coerce_merge_keys(self) -> None:
                     rk.dtype
                 ):
                     ct = find_common_type([lk.dtype, rk.dtype])
-                    rk = ct.construct_array_type()._from_sequence(rk)
+                    if is_extension_array_dtype(ct):
+                        rk = ct.construct_array_type()._from_sequence(rk)  # type: ignore[union-attr]  # noqa: E501
+                    else:
+                        rk = rk.astype(ct)  # type: ignore[arg-type]
                 elif is_extension_array_dtype(rk.dtype):
                     ct = find_common_type([lk.dtype, rk.dtype])
-                    lk = ct.construct_array_type()._from_sequence(lk)
+                    if is_extension_array_dtype(ct):
+                        lk = ct.construct_array_type()._from_sequence(lk)  # type: ignore[union-attr]  # noqa: E501
+                    else:
+                        lk = lk.astype(ct)  # type: ignore[arg-type]
 
                 # check whether ints and floats
                 if is_integer_dtype(rk.dtype) and is_float_dtype(lk.dtype):