On July 22 I wrote:

> I see three ways to go here.
> 
> 1. We change eql, memql, sxhash-eql, etc. to treat all NaNs alike, so that there 
> is just one NaN value from the Lisp point of view. We also change 'format', 
> 'copysign' etc. to ignore a NaN's sign, and look through any other use of 
> floating-point values to make sure that NaN signs are ignored.
> 
> 2. We change eql, memql, sxhash-eql, etc. so that only the sign (not the 
> significand) of a NaN is looked at, so that there are just two NaN values from 
> the Lisp point of view.
> 
> 3. We alter 'read', 'format' etc. to read and generate NaN significands. For 
> example, (format "%s" NaN) could return "0.1e+NaN" if the significand's 
> low-order bit was set.
> 
> (1) sounds too drastic, as the sign of a NaN can be useful in some cases and 
> Emacs has long provided for obtaining the sign of a NaN. Although either (2) or 
> (3) would be OK, I'm inclined to go for (3) as I expect it would be a bit 
> cleaner and more useful.


No further comment on this, and I went with (3) and installed into master the 
attached patch to do something along those lines on platforms that have 
ieee754.h (mostly GNUish platforms, I expect). Other platforms are unaffected.