Personally I'm not married to the current grammar, but we do need a
plausible alternative, and we need it soon.

An alternative syntax needs to have:
- an obvious terminator (a comma is not that, especially for assert)
- somewhere to do the preamble (pre:, post r:, assert: )
- not be current valid syntax.

Thinking out loud here, but could braces work?

auto f(auto const x, int const y) noexcept -> void
    requires integral<decltype(x)>
    pre {
         x > 0;
         y > x;
    }
    pre(axiom) {
        {x % 2 == 0};
    }
    post(audit, new) [r=return, x, y] { // seems like lambda-like captures
fit in
        {r % x == 0};
        {r % y == 0};
    }
{
/* function body */
   assert {
      { x > 0 };
   };
}

That looks pretty much like the grammar for the "requires" expression
except with runtime values. Hm.....

On Wed, Sep 22, 2021 at 5:48 PM Aaron Ballman via SG21 <
sg21_at_[hidden]> wrote:

> On Wed, Sep 22, 2021 at 11:06 AM Jens Maurer via SG21
> <sg21_at_[hidden]> wrote:
> >
> > On 22/09/2021 12.19, Andrzej Krzemienski via Liaison wrote:
> > >
> > >
> > > śr., 22 wrz 2021 o 09:42 Ville Voutilainen via Liaison <
> liaison_at_[hidden] <mailto:liaison_at_[hidden]>> napisał(a):
> > >
> > > On Wed, 22 Sept 2021 at 00:02, Jens Maurer via Liaison
> > > <liaison_at_[hidden] <mailto:liaison_at_[hidden]>>
> wrote:
> > > > (Personally, I think contracts should be a first-level
> > > > language feature that should not be hidden inside an
> > > > attribute-looking syntax atrocity. At least in C++,
> > > > the space where they are does allow for context-sensitive
> > > > keywords without much hassle; cf. override and final.)
> > >
> > > Well, yeah.. if we were to entertain a contract declaration
> preceding
> > > the decl-specifier,
> > > so that it could do forward-lookup for the parameters, then the
> case
> > > for an attribute-like
> > > syntax would be relatively strong. If the contract declaration has
> to
> > > appear where context-sensitive
> > > keywords appear, then why not use a context-sensitive keyword?
> > >
> > >
> > > The current contracts proposal (P2388R2 <
> http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2021/p2388r2.html>) as
> well as C++20 contracts offer(ed) three new annotations:
> > > * preconditions and postconditions that appear in function
> declarations,
> >
> > ... in a place where we have good experience with context-sensitive
> > keywords.
> >
> > > * an assertion that appears in the function body.
> > >
> > > This third kind is in the position of a regular statement inside a
> block scope, so a context-sensitive keyword will not work; unless we were
> to drop these assertions or treat them in a different, irregular way.
> >
> > Well, we could do
> >
> > pre: conditional-expression
> > post identifier: conditional-expression
> > assert: conditional-expression // in a block
> >
> > and the only potential conflict would be with a user-defined label
> "assert".
> >
> > (The colon should be a pretty good disambiguator for pre and post;
> > if that's not enough, we can require a logical-or-expression and/or
> > add a comma to separate contracts in declarations.)
> >
> > > Also a natural future extension of the currently proposed contracts is
> class invariants, which would also appear as a declaration in class-scope,
> so no room for context-sensitive keywords.
> >
> > With the colon, I'm not seeing a serious problem.
>
> This sort of syntax would alleviate my personal concerns with the
> proposed syntax.
>
> In terms of whether this would be palatable to WG14, it's a bit less
> clear. C has no context sensitive keywords and I have no memory of
> WG14 discussions to add any, so I don't have much experience to draw
> from. However, I think context sensitive keywords are a reasonable
> idea worth seriously exploring as it seems at least plausible (we have
> plenty of implementation and usage experience with override and final
> in C++ that we can point to as prior art that's mildly C related, but
> if anyone knows of a context-sensitive keyword in a C implementation,
> that would strengthen the case in WG14 greatly).
>
> ~Aaron
>
> >
> > Jens
> >
> > _______________________________________________
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