RE: problems with face-spec-reset-face and set-face-attribute

[Drew Adams]

        > My comment about a possible bug specific to :inherit was because
        > evaluating the same expression, namely
        >
        >   (set-face-attribute 'completions-first-difference nil
              ATTR 'unspecified)
        >
        > when ATTR is different from :inherit, e.g. :background or
           :slant, does
        > work as Drew expected, and as the doc strings say it should.

        In my tests this doesn't work for other attributes too.  Using

        (set-face-attribute 'highlight nil :background 'unspecified)
        (set-face-attribute 'italic nil :slant 'unspecified)

        in the above recipe produces the same result, i.e. doesn't
        change the default for new frames.

    I think I understand why.

    Emacs actually has two places to get the face specifications
    for new frames.  There is face-new-frame-defaults, and there
    is whatever the defface specified.

    The face-new-frame-defaults are merged in by
    Finternal_merge_in_global_face, and normally override what
    the defface specified.  But this happens only when the value
    in face-new-frame-defaults is not `unspecified'.

    In other words, setting an attribute to `unspecified'
    has an inconsistent meaning.  For an existing frame, it overrides
    both the face-new-frame-defaults and the defface.  But when
    applied to the "default for new frames", it really means that
    face-new-frame-defaults should no longer override the defface.

    This is rather ugly, but I am not sure what to clean it up with.

That helps me understand. It seems very complex. I think I follow you, but I
don't understand it well enough to propose any solution. I think part of
what confuses me is the notion of a default value for a face (e.g. for
future frames). I'm still a bit confused about the behavior of :inherit and
nil wrt defining/undefining such a default value - and the role, if any, of
`unspecified' in this regard. (That probably makes no sense, but it
indicates my confusion.)

Anyway, given that I'm a bit confused, and you seem to understand the
problem I raised, I'll bow out now. Thanks for looking into this.