bug#27103: Should show-trailing-whitespace highlight specified spaces?

[Eli Zaretskii]

> Cc: 27103@debbugs.gnu.org
> From: Clément Pit--Claudel <clement.pitclaudel@live.com>
> Date: Sat, 27 May 2017 15:52:27 -0400
> 
> Would the same caveat apply to the creation of a new 'left-fringe text 
> property, such that instead of (concat "A" (propertize "_" 'display 
> '(left-fringe filled-square))) one could write (propertize "A" 'left-fringe 
> 'filled-square)?

I'm not sure I understand why the former uses 'concat', while the
latter doesn't.  So perhaps I'm missing the point, because otherwise
this looks like just "syntactic sugar", is it?