🚀 go-pugleaf

In article <Pine.ULT.3.91.951017124752.2343A-100000@mars.dsv.su.se>, Jens M Andreasen <jens-and@dsv.su.se> writes:
>On 16 Oct 1995, Paul Long wrote:
>> Is an implementation required to generate a diagnostic for this declaration?
>>    static int i = 1 ? 3 : (1, 2);

> After parsing your fancy expression, the compiler ends up with something 
>like: static i = 3; 
>Looks legal to me.

No, after parsing the expression, the compiler ends up with something like:
1 ? 3 : (1, 2);
The question is not the result of evaluating the expression, the question is
whether the expression is permitted.  And the problem is that the standard
requires a diagnostic for violation of a constraint (which this is) and the
standard permits implementations to define extensions in cases of undefined
behavior (which this is, i.e. the implementation can issue a diagnostic and
proceed to compute a value), but the standard adds a sentence saying that
implementations can accept additional forms of constant expression.  What
does this nonsense mean?

If the extra sentence means that implementation extensions are permitted in
a case of undefined behavior (but not forgetting a diagnostic) then it is
a no-op.  If the extra sentence means that implementation extensions are
permitted in a case of undefined behavior (including forgetting a diagnostic)
then it contradicts the part of the standard that requires diagnostics for
violations of constraints.  I think someone once posted that information had
been received from the committee, suggesting that their intention was that
the extra sentence override the general rule so that a diagnostic is not
required when an implementation extension accepts an otherwise invalid
constant expression.  This also doesn't make a lot of sense because the
committee could have put the restrictions in a semantics paragraph instead
of a constraints paragraph, achieving the same result and still not requiring
the extra sentence.

I think the answer to Mr. Long's question is:  No one knows.
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