Re: [V1][PATCH 1/3] Provide element_count attribute to flexible array member field (PR108896)

[Joseph Myers <joseph@codesourcery.com>]

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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
joseph@codesourcery.com