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Re: [wg14/wg21 liaison] [isocpp-sg21] Telecon to review P2388R1 Minimum Contract Support: either Ignore or Check_and_abort

From: Aaron Ballman <compatibility.sg.chair_at_[hidden]>
Date: Mon, 20 Sep 2021 11:57:08 -0400
On Mon, Sep 20, 2021 at 11:15 AM Ville Voutilainen via Liaison
<liaison_at_[hidden]> wrote:
> On Mon, 20 Sept 2021 at 18:02, Aaron Ballman via SG21
> <sg21_at_[hidden]> wrote:
> > There are compilers for which the implementation strategy for all
> > attributes is to note the [[ that opens an attribute specifier, eat
> > all balanced tokens up to the closing ]], and diagnose the entire bit
> > as "attribute ignored" (as a warning). You cannot do that with
> > contracts because a correct program does not remain correct when the
> > contract is ignored. A program can correctly rely on the contract
> > violation handler executing, and that's not the case with this
> > implementation strategy.
> That's not a conforming strategy. A compiler needs to parse the
> attribute grammar, and that grammar doesn't
> allow arbitrary tokens as a balanced-token-sequence outside an
> attribute-argument-clause, as specified
> in in the C working draft.

It is a conforming strategy because of the laxity in what diagnostics
are required for conformance. "attribute ignored" is a low-quality
diagnostic for [[12]] but is still a conforming implementation. The
issue with contracts is that there are additional semantics that can't
be ignored.


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Received on 2021-09-20 10:58:33