On Fri, 16 Jun 2023, Martin Uecker via Gcc-patches wrote:
> > Note that no expressions can start with the '.' token at present. As soon
> > as you invent a new kind of expression that can start with that token, you
> > have syntactic ambiguity.
> >
> > struct s1 { int c; char a[(struct s2 { int c; char b[.c]; }) {.c=.c}.c]; };
> >
> > Is ".c=.c" a use of the existing syntax for designated initializers, with
> > the first ".c" being a designator and the second being a use of the new
> > kind of expression, or is it an assignment expression, where both the LHS
> > and the RHS of the assignment use the new kind of expression? And do
> > those .c, when the use the new kind of expression, refer to the inner or
> > outer struct definition?
>
> I would treat this is one integrated feature. Essentially .c is
> somthing like this->c for the current struct for designated
> initializer *and* size expressions because it is semantically
> so close. In the initializer I would allow only
> the current use for designated initialization for all names of
> member of the currently initialized struct, so .c = .c would
> be invalid. It should never refer to the outer struct if there
I'm not clear on what the intended disambiguation rule here is, when "."
is seen in initializer list context - does this rule depend on whether the
following identifier is a member of the struct being initialized, so
".c=.c" would be OK above if the initialized struct didn't have a member
called c but the outer struct definition did? That seems like a rather
messy rule. And does "would allow only" apply other than in the ambiguous
context? That seems to be implied by ".c=.c" being invalid above, because
to make it invalid you need to disallow the new construct being used for
the second .c, not just make the first .c interpreted as a designator.
Again, this sort of thing needs a detailed written specification, with
multiple iterations discussed among different implementations. The above
paragraph doesn't make clear to me any of: the disambiguation rules; what
is allowed in what context; how name lookup works (consider tricky cases
such as a reference to an identifier declared *later* in the same struct,
possibly in the context of C2x tag compatibility where a previous
definition of the struct is visible); when these expressions get
evaluated; what the underlying principles are behind those choices.
Using a token (existing or new) other than '.' - one that doesn't
introduce ambiguity in any context where expressions can be used - would
help significantly, although some of the issues would still apply.
--
Joseph S. Myers
[email protected]
