bug#39265: 26.3; format-mode-line with FACE argument

[ynyaaa]

#'format-mode-line assumes all characters of a string have a same 'face
property value and checks only the first character.

  (format-mode-line (concat "A" (propertize "XYZ" 'face 'region)) 'highlight)
  => #("AXYZ" 0 1 (face highlight) 1 4 (face highlight))

#'format-mode-line merges the 'face property value of the first
character of a string with FACE argument. It is not ducumented at all.

  (format-mode-line (propertize "ABC" 'face 'region) 'highlight)
  => #("ABC" 0 3 (face (region highlight)))

#'format-mode-line called with FACE=t should use 'mode-line or
'mode-line-inactive instead of 't, but it treats 't as a face.

  (format-mode-line "ABC" t)
  => #("ABC" 0 3 (face t))


In GNU Emacs 26.3 (build 1, x86_64-w64-mingw32)
 of 2019-08-29 built on CIRROCUMULUS
Repository revision: 96dd0196c28bc36779584e47fffcca433c9309cd
Windowing system distributor 'Microsoft Corp.', version 10.0.18363
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Configured using:
 'configure --without-dbus --host=x86_64-w64-mingw32
 --without-compress-install 'CFLAGS=-O2 -static -g3''

Configured features:
XPM JPEG TIFF GIF PNG RSVG SOUND NOTIFY ACL GNUTLS LIBXML2 ZLIB
TOOLKIT_SCROLL_BARS THREADS LCMS2

Important settings:
  value of $LANG: JPN
  locale-coding-system: cp932

Major mode: Lisp Interaction

Minor modes in effect:
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  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: t

Load-path shadows:
None found.

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backquote threads w32notify w32 lcms2 multi-tty make-network-process
emacs)

Memory information:
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