bug#20537: 25.0.50; `compose-string-internal` does not work properly on OSX.

[Eli Zaretskii]

> Date: Sat, 09 May 2015 11:27:15 -0700
> From: Keith David Bershatsky <esq@lawlist.com>
> 
> The function `compose-string-internal` does not work properly when running 
> Emacs built on OSX.  The following example works on Windows, but not OSX -- 
> it "should be" the letter A with a pipe running through the center of the 
> letter A.  Depending upon the arguments passed to `compose-string-internal` 
> when using Emacs on OSX, the pipe is either displayed to the right (at 
> various distances) or is not visible at all -- it never displays "on top of" 
> the target character.
> 
> (defun test-fn ()
> (interactive)
>   (message (compose-string-internal "A|" 0 (length "A|"))))

Character composition depends on the font driver doing TRT.  Which
font driver does your Emacs use, and does that driver support complex
text layout?