bug#20707: [PROPOSED PATCH] Use curved quoting in C-generated errors

[Paul Eggert <eggert@cs.ucla.edu>]

Alan Mackenzie wrote:
> I'd challenge you to argue that the
> following glyph for 0x60 originated as left curly quote:

The history doesn't matter that much.  What's more important is how Emacs works 
now.  Today's displays mostly show curved quotes as curved quotes, show 
apostrophe as a straight vertical quote, and show grave accent as a straight 
slanted accent, and Emacs doesn't look good on these displays.  That's the bug 
that needs to be fixed.

> By "data points" is meant those who use Emacs on the (Linux) console.  I
> repeat, the only such user who's expressed a view on your proposed change
> is me.

I use Emacs on the Linux console occasionally.  It works adequately in the 
latest master.  So we have at least two data points.

> Curly quotes are not currently in use in (released) Emacs

Yes they are, when reading the documentation.

> What's the flag to turn it off called?

Sorry, there's no flag.  We couldn't just "turn it off", as doc strings will 
contain curved quotes and these need to be parsed regardless of user display 
preferences.  I suppose there could be an option to transliterate them to 
straight quotes before insertion into the *Help* buffer, for users who prefer 
that.  But this should be done regardless of whether the source code uses curved 
quotes.  And it shouldn't be done if the source uses escaped characters.

> what other strings other than doc strings might want this change?

Mostly diagnostics.

> I can't recall seeing any instance of Lisp or C code processing doc
> strings, or `error' arguments in any way that would make a difference.
> Can you cite a specific example?

Sure, see the latest commit to the master: commit 
8afef016e2bc413c7a4913554ea6ed2c55474de6.  There's lots more code like that.