id,node_id,number,title,user,state,locked,assignee,milestone,comments,created_at,updated_at,closed_at,author_association,active_lock_reason,draft,pull_request,body,reactions,performed_via_github_app,state_reason,repo,type
130767608,MDExOlB1bGxSZXF1ZXN0NTgwMjY4NjY=,743,BUG: reindexing an array with complex values should keep dtype=complex,11950875,closed,0,,,6,2016-02-02T18:11:48Z,2016-02-04T23:13:09Z,2016-02-04T23:12:37Z,CONTRIBUTOR,,0,pydata/xarray/pulls/743,"Fixes #738 

I have added a simple behaviour for complex numbers in _maybe_promote. This fix means that  _maybe_promote does indeed return (dtype, np.nan + 0j) for complex dtypes, and the complex -> object conversion shown in #738 does not occur.
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