how to auto-indent both Tab and Space indented lines?

In my files, some lines are indented with Tab's, while others uses
spaces.
If I set cindent or smartindent, new lines after one set of these
lines cannot be auto-indented correctly.
So I have to set autoindent and add or remove indents manually when
necessary.
Cannot VIM auto discern previous line's indent style to auto indent a
new line?

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