Path: news.eternal-september.org!eternal-september.org!.POSTED!not-for-mail
From: Keith Thompson <Keith.S.Thompson+u@gmail.com>
Newsgroups: comp.lang.c
Subject: Re: "A diagram of C23 basic types"
Date: Mon, 14 Apr 2025 04:08:38 -0700
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Tim Rentsch <tr.17687@z991.linuxsc.com> writes:
> Keith Thompson <Keith.S.Thompson+u@gmail.com> writes:
>> Tim Rentsch <tr.17687@z991.linuxsc.com> writes:
>> [...]
>>
>>> Trailing commas in argument lists and/or parameter lists
>>> could be accepted as an extension, even without giving a
>>> diagnostic as I read the C standard, but implementations
>>> are certainly within their rights to reject them.
>>
>> I believe a diagnotic is required.
>>
>> C17 5.1.1.3:
>>
>>     A conforming implementation shall produce at least one
>>     diagnostic message (identified in an implementation-defined
>>     manner) if a preprocessing translation unit or translation
>>     unit contains a violation of any syntax rule or constraint,
>>     even if the behavior is also explicitly specified as undefined
>>     or implementation-defined.
>>
>> A trailing comma on an argument or parameter list is a violation
>> of a syntax rule.
>
> I believe a diagnostic is not required, because the C standard
> explicitly allows extensions.  If such diagnostics were required
> even for constructions that are part of extensions, then there is no
> reason to allow extensions, because whatever behavior is desired
> could be done anyway, under the freedom granted by undefined
> behavior.  It would be stupid to explicitly grant permission to do
> something if it could be done anyway without the permission.  And
> the people who wrote the C standard are not stupid.

I believe that the fact that a conforming implementation provides an
extension does not relieve it of its responsibility to diagnose syntax
errors and constraint violations.

The authors of both gcc and clang appear to agree.  A concrete example:
gcc has an extension (which clang also provides) allowing the middle
operand of a conditional operator to be omitted, so that `x ? : y` is
equivalent to `x ? x : y`, except that x is evaluated only once.

By default, use of this extension (which would be a syntax
error if the extension did not exist) is not diagnosed; neither
compiler is conforming by default.  With "-std=c17 -pedantic" or
"-std=c17 -pedantic-errors", both issue a warning or error message,
respectively.

-- 
Keith Thompson (The_Other_Keith) Keith.S.Thompson+u@gmail.com
void Void(void) { Void(); } /* The recursive call of the void */