* lisp/gnus/gnus-sum.el (gnus-tmp-thread): New var
The recent change to use lexical-binding introduced a bug because gnus-summary-line-format-alist refers to variable `thread` which is now lexical and hence not available there any more. Replace it with a dynamically scoped var `gnus-tmp-thread` as was already done with all other vars. (gnus-summary-line-format-alist): Use it instead of `thread`. (gnus-summary-prepare-threads): Bind it around call to (eval gnus-summary-line-format-spec).
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