Bug #64232 turned out to be invalid; what I wanted was already the case.

This matter was explored more deeply by Tadziu and me in a thread on the groff list.

https://lists.gnu.org/archive/html/groff/2024-01/msg00019.html
https://lists.gnu.org/archive/html/groff/2024-01/msg00032.html
https://lists.gnu.org/archive/html/groff/2024-01/msg00040.html
https://lists.gnu.org/archive/html/groff/2024-01/msg00041.html
https://lists.gnu.org/archive/html/groff/2024-01/msg00048.html
https://lists.gnu.org/archive/html/groff/2024-01/msg00050.html

The most pertinent part for this ticket is likely the following.

...[N]othing in eqn documents its sensitivity to the font mounting
order when determining the typefaces to be used.  In fact that
sensitivity overrides explicit configuration!

Source:
$ cat ATTIC/to-slant-or-not-to-slant.roff
.\" "letter" means "slanted" in GNU eqn
.\" This is actually redundant with the default configuration.
.EQ
chartype "letter" \[*a]\[*b]\[*g]\[*d]\[*e]\[*z]
chartype "letter" \[*y]\[*h]\[*i]\[*k]\[*l]\[*m]
chartype "letter" \[*n]\[*c]\[*o]\[*p]\[*r]\[*s]
chartype "letter" \[*t]\[*u]\[*f]\[*x]\[*q]\[*w]
.EN
.fp 5 S
.fp 6 SS
\(*A\(*a \(-> upright lowercase alpha
.br
.EQ
sin ( 2 theta ) ~ = ~ 2 ~ sin theta cos theta
.EN
.sp
.fp 5 SS
.fp 6 S
\(*A\(*a \(-> slanted lowercase alpha
.br
.EQ
sin ( 2 theta ) ~ = ~ 2 ~ sin theta cos theta
.EN

Rendering:
[see attached image]

> Example:
>
>   .fp 5 S
>   .fp 6 SS
>   \(*A\(*a --> upright lowercase alpha.
>   .fp 5 SS
>   .fp 6 S
>   \(*A\(*a --> slanted lowercase alpha.

This is fine, but it's raw *roff, not eqn input.  I am concerned with
eqn output, and nothing I propose would break your exhibit above.

At 2024-01-10T02:10:12+0100, Tadziu Hoffmann wrote:
> > I never knew this.  Where is the reference please?
> > I would like to mention this in my new EQN manual.
>
> The Troff User's Manual (section 2, "Font and Character Size
> Control") says:
>
>   The troff character set is defined by a description file
>   specific to each output device.  There are normally several
>   regular fonts and one or more special fonts.
>
>   Troff begins execution by reading information for a set of
>   defaults fonts, said to be mounted; conventionally, the first
>   four are Times Roman (R), Times Italic (I), Times Bold (B),
>   and Times Bold Italic (BI), and the last is a Special font (S)
>   containing miscellaneous characters.  The set of fonts and
>   positions is determined by the device description file.
>
>   It is not necessary to change to the Special font;
>   characters on that font are automatically handled as if
>   they were physically part of the current font.  The Special
>   font may actually be several fonts; the name S is reserved
>   and is generally used for one of these.  All special fonts
>   must be mounted after regular fonts.
>
> However, it does not explicitly say that the special fonts
> are searched in mount-position order.  (I think it is a
> reasonable assumption, but I may be biased.)
>
> The groff Info file (section 5.17.4, "Using Symbols") is more
> specific:
>
>   Here are the exact rules how 'gtroff' searches a given symbol:
>
>    [... current font and explicit declarations using
>      .char and .fspecial ...]
>
>    * As a last resort, consult all fonts loaded up to now for
>      special fonts and check them, starting with the lowest
>      font number.  [...]
>
> Since both S and SS contain lowercase Greek characters, placing
> SS before S will result in gtroff picking the slanted alpha for
> \(*a, whereas placing S before SS with pick the upright alpha.
> (Unless of course the current font also contains a \(*a character,
> in which case this will be used, or any other font declared
> as special and containing \(*a is mounted before S and SS.)

I don't propose to change any of this.  It seems to work well and can be
apprehended by users.

...

[3] This in turn means that the definitions of the eqn Greek-letter-name
    macros will _have_ to come out of the lexer and be put in
    eqn-device-specific startup logic--in eqnrc, say--so that the code
    can be appropriately conditionalized.  Only the "ps" device has
    (though, soon, "pdf" will have) a font called "SS" by default.

    So I envision something like this in eqnrc:

ifdef utf8 ! define Alpha \*(A !
ifdef utf8 ! define ALPHA \*(A !
ifdef utf8 ! define alpha \fI\*(a\fP !

ifdef ps|pdf ! define Alpha \*(A !
ifdef ps|pdf ! define ALPHA \*(A !
ifdef ps|pdf ! define alpha \f(SS\*(a\fP !

   ...and so on for the other 23 Greek letters.

The dvi, html, pdf, and ps output drivers all set Greek lowercase Greek letters in italics.

We should do the same for nroff mode, given the known equation context.