BUG: concat of Series of EA and other dtype fails
Closes #20832
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20840?src=pr&el=h1) Report
> Merging [#20840](https://codecov.io/gh/pandas-dev/pandas/pull/20840?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/6cacdde5630c593999059833b516e1fec60aaf72?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/20840/graphs/tree.svg?token=eZ4WkYLtcO&width=650&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/20840?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #20840      +/-   ##
==========================================
+ Coverage   91.77%   91.78%   +<.01%     
==========================================
  Files         153      153              
  Lines       49280    49313      +33     
==========================================
+ Hits        45229    45261      +32     
- Misses       4051     4052       +1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.17% <100%> (ø)` | :arrow_up: |
| #single | `41.88% <100%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/20840?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/dtypes/concat.py](https://codecov.io/gh/pandas-dev/pandas/pull/20840/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2NvbmNhdC5weQ==) | `99.18% <100%> (ø)` | :arrow_up: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/20840/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `84.59% <0%> (-0.21%)` | :arrow_down: |
| [pandas/core/arrays/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/20840/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2Jhc2UucHk=) | `83.95% <0%> (ø)` | :arrow_up: |
| [pandas/api/extensions/\_\_init\_\_.py](https://codecov.io/gh/pandas-dev/pandas/pull/20840/diff?src=pr&el=tree#diff-cGFuZGFzL2FwaS9leHRlbnNpb25zL19faW5pdF9fLnB5) | `100% <0%> (ø)` | :arrow_up: |
| [pandas/core/series.py](https://codecov.io/gh/pandas-dev/pandas/pull/20840/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc2VyaWVzLnB5) | `93.99% <0%> (ø)` | :arrow_up: |
| [pandas/core/internals.py](https://codecov.io/gh/pandas-dev/pandas/pull/20840/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW50ZXJuYWxzLnB5) | `95.59% <0%> (+0.01%)` | :arrow_up: |
| [pandas/core/indexing.py](https://codecov.io/gh/pandas-dev/pandas/pull/20840/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhpbmcucHk=) | `93.14% <0%> (+0.05%)` | :arrow_up: |
| [pandas/core/dtypes/missing.py](https://codecov.io/gh/pandas-dev/pandas/pull/20840/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL21pc3NpbmcucHk=) | `92.94% <0%> (+0.08%)` | :arrow_up: |
| [pandas/core/algorithms.py](https://codecov.io/gh/pandas-dev/pandas/pull/20840/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYWxnb3JpdGhtcy5weQ==) | `94.48% <0%> (+0.08%)` | :arrow_up: |
| [pandas/core/dtypes/cast.py](https://codecov.io/gh/pandas-dev/pandas/pull/20840/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2Nhc3QucHk=) | `88.06% <0%> (+0.2%)` | :arrow_up: |
| ... and [1 more](https://codecov.io/gh/pandas-dev/pandas/pull/20840/diff?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20840?src=pr&el=continue).
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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20840?src=pr&el=footer). Last update [6cacdde...964a5ff](https://codecov.io/gh/pandas-dev/pandas/pull/20840?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 I may have missed some, only glanced briefly in extension/base/reshape.py

On Sat, Apr 28, 2018 at 7:07 AM, Joris Van den Bossche <
notifications@github.com> wrote:

> *@jorisvandenbossche* commented on this pull request.
> ------------------------------
>
> In pandas/tests/extension/base/reshaping.py
> <https://github.com/pandas-dev/pandas/pull/20840#discussion_r184851485>:
>
> > @@ -64,6 +64,11 @@ def test_concat_mixed_dtypes(self, data):
>          expected = pd.concat([df1.astype('object'), df2.astype('object')])
>          self.assert_frame_equal(result, expected)
>
> +        result = pd.concat([df1['A'], df2['A']])
>
> Hmm, thought we already had some, but indeed, apparently not. Will add
> some.
>
> —
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