Point-Char: |

Name: fill-region-as-paragraph-default
Code:
  (lambda ()
    (setq-local fill-column 35)
    (fill-region-as-paragraph-default
     (point)
     (progn
       (goto-char (point-max))
       (forward-line -1)
       (beginning-of-line)
       (point))))

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Lorem ipsum dolor sit amet, consectetur adipiscing elit, s|ed do
eiusmod tempor.  Incididunt ut labore et dolore magna aliqua. Ut enim
ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut.

Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do
eiusmod tempor.  Incididunt ut labore et dolore magna aliqua. Ut enim
ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut.

Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do
eiusmod tempor.  Incididunt ut labore et dolore magna aliqua. Ut enim
ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut.
=-=
Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed
do eiusmod tempor.  Incididunt ut
labore et dolore magna aliqua. Ut
enim ad minim veniam, quis nostrud
exercitation ullamco laboris nisi
ut.  Lorem ipsum dolor sit amet,
consectetur adipiscing elit, sed do
eiusmod tempor.  Incididunt ut
labore et dolore magna aliqua. Ut
enim ad minim veniam, quis nostrud
exercitation ullamco laboris nisi
ut.  Lorem ipsum dolor sit amet,
consectetur adipiscing elit, sed do
eiusmod tempor.  Incididunt ut
labore et dolore magna aliqua. Ut
enim
ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut.
=-=-=

Code:
  (lambda ()
    (fill-region-as-paragraph-default (point-min) (point-max)))

Name: fill-region-as-paragraph-default - merge lines

=-=
abc def.
abc def.
=-=
abc def.  abc def.
=-=-=

Name: fill-region-as-paragraph-default - merge lines with wide characters

=-=
あいうえお。
あいうえお。
=-=
あいうえお。あいうえお。
=-=-=

Code:
  (lambda ()
    (setq-local sentence-end-double-space nil)
    (fill-region-as-paragraph-default (point-min) (point-max)))

Name: fill-region-as-paragraph-default - single space - merge lines

=-=
abc def.
abc def.
=-=
abc def. abc def.
=-=-=

Name: fill-region-as-paragraph-default - single space - merge lines with wide characters

=-=
あいうえお。
あいうえお。
=-=
あいうえお。あいうえお。
=-=-=

Name: fill-region-as-paragraph-default - fill-separate-heterogeneous-words-with-space (Bug#79575)
Code:
  (lambda ()
    (setq-local fill-separate-heterogeneous-words-with-space t)
    (fill-region-as-paragraph-default (point-min) (point-max)))

=-=
abc def.
あいうえお。
=-=
abc def.  あいうえお。
=-=-=

Name: fill-region-as-paragraph-default - single space - fill-separate-heterogeneous-words-with-space
Code:
  (lambda ()
    (setq-local fill-separate-heterogeneous-words-with-space t)
    (setq-local sentence-end-double-space nil)
    (fill-region-as-paragraph-default (point-min) (point-max)))

=-=
abc def.
あいうえお。
=-=
abc def. あいうえお。
=-=-=