REGR: assignment of pd.NA with enlargement gives object dtype with IntegerArray

[simonjayhawkins]

Pandas version checks

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Reproducible Example

# gets upcast to object
df = pd.DataFrame({"a": [1, 2, 3]}, dtype="Int64")
df.loc[4] = pd.NA

Issue Description

xref #32346 (comment), #47214

Expected Behavior

In [1]: df = pd.DataFrame({"a": [1, 2, 3]}, dtype="Int64")
...: df.loc[4] = pd.NA

In [2]: df
Out[2]:
a
0 1
1 2
2 3
4

In [3]: df.dtypes
Out[3]:
a Int64
dtype: object

In [4]: pd.version
Out[4]: '1.3.5'

Installed Versions

.

[simonjayhawkins]

first bad commit: [084c543] BUG/API: concat with empty DataFrames or all-NA columns (#43507)

cc @jbrockmendel

[jbrockmendel]

Looks like we get to _setitem_with_indexer_missing with value=pd.NA and end up doing value = Series(value, index=self.obj.columns, name=indexer), which comes back as object-dtype. the concat that follows preserves object dtype.

We may need to special-case pd.NA here, or otherwise avoid getting object-dtype back.

[jbrockmendel]

Does #47372 include a test for this?