The hyphenchar ligature in T1

[Lars Hellström <Lars.Hellstrom@math.umu.se>]

For a while now, I have been curious about the

   hyphen (\char45)  +  hyphenchar (\char127)  ->  hyphenchar

ligature in T1 encoded fonts. I have some questions related to this
ligature, that I hope someone answer:

1. When is this ligature ever used?

I see no frequent case when any command or combination of explicit
characters put a hyphenchar after a hyphen (you can always write -\-, of
course, but I doubt anyone do this frequently). My best guess is that there
is something in the hyphenation patterns which makes this ligature
realistic, but I don't know.

2. Would anything be different if the ligature was
   hyphen  +  hyphenchar  ->  hyphen
instead?

If hyphen and hyphenchar are the same glyph then the printed output will
look the same in both cases, and if they are different then would it not be
more reasonable that the output contains the explicit hyphen that the user
actually typed instead of a hyphenchar? I suspect that it may be the case
that TeX does not use the same penalty for a linebreak after a hyphen in
the two cases (but I haven't bothered to check).

Lars Hellström