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From: Keith Thompson <Keith.S.Thompson+u@gmail.com>
Newsgroups: comp.lang.c
Subject: Re: clarification about enums in C
Date: Thu, 27 Feb 2020 06:33:07 -0800
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David Brown <david.brown@hesbynett.no> writes:
[...]
>> enum enum_t {
>>  uns = (unsigned)-1,
>>  neg = -1,
>> };
>> Both in gcc and in clang uns has type 'unsigned int' rather than 'int'.
>> And indeed it differs from type of 'enum_t' which in ths case is 'int'.
>
> This is an extension that these tools support - as do many compilers.
> If you want to ensure that you stick to standards-only code, use
> "-Wpedantic" (along with your choice of -std=c11, -std=c99 or -std=c90),
> and pay attention to the warnings.  The C standard does not say how
> "uns" should be treated, gcc's extensions do the "obvious" thing.

Ah, but gcc doesn't print a diagnostic with "-std=c11 -Wpedantic", even
though the code violates a constraint.

N1570 6.7.2.2p2:

    The expression that defines the value of an enumeration constant
    shall be an integer constant expression that has a value
    representable as an int.

Even my copy of gcc built from source that calls itself
    gcc (GCC) 10.0.0 20200111 (experimental)
fails to diagnose it.

A recent version of clang does print a diagnostic.

-- 
Keith Thompson (The_Other_Keith) Keith.S.Thompson+u@gmail.com
Working, but not speaking, for Philips Healthcare
void Void(void) { Void(); } /* The recursive call of the void */