Problem with narrow vs condensed fonts

[Stefan Monnier]

My Emacs by default fails to show me the variable-pitch face.
I tracked it down to the following problem:

My default font is misc-fixed-semicondensed-13, and on my system I have
some adobe-helvetica-narrow fonts.  Since semicondensed=87 and
narrow=75 (and normal is 100), Emacs decided to prefer
adobe-helvetica-narrow over adobe-helvetica-normal.  But by the time
this choice is made we don't have XLFD font names any more but font
entities, so the spec just say "swidth=75" and when we try to open the
font that we just listed this fails because 75 is translated back to
"condensed" rather than to "narrow".

In essence font-swidth-table needs to be bijective but isn't.
By changing "narrow"'s setting from 75 to 76 (so it doesn't get the
same value as any other any more) the problem disappears (and another
problem shows up: I now get this narrow font where I'd prefer the
normal font since helvetica is already pretty narrow).

Does this make sense?  Should we fix font-swidth-table and friends (and
change internal-set-font-style-table to check that the tables are indeed
bijective)?


        Stefan