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From: Tim Rentsch <tr.17687@z991.linuxsc.com>
Newsgroups: comp.lang.c
Subject: Re: memcpy not memcpying?
Date: Mon, 04 Nov 2019 20:58:51 -0800
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Keith Thompson <kst-u@mib.org> writes:

> Ben Bacarisse <ben.usenet@bsb.me.uk> writes:
> [...]
>
>> I think I might use _Generic here because checking the size is not
>> always going to be enough:
>>
>>      _Static_assert(
>>           _Generic(i, int: 1, default: 0),
>>           "Check this because symbolcount_t is not compatible with int."
>>      );
>>
>> This works (in both clang and gcc) with -std=c11 -pedantic, but I am not
>> sure that _Generic /must/ be allowed here.  It would be a shame if
>> generic selections were not considered to be standard constant
>> expressions.  Have I missed something in the standard?
>
> I think it's covered in 6.6, which has this contraint in p2:
>
>     Constant expressions shall not contain assignment, increment,
>     decrement, function-call, or comma operators, except when they
>     are contained within a subexpression that is not evaluated.
>
> Which implies, I think, that a generic selection is a constant
> expression if the result expression is a constant expression,
> even if the other subexpressions are not.

What a strange inference.  The quoted sentence can tell us only when
an expression is not a constant expression, never when it is a
constant expression.

Besides which, a _Static_assert needs not just a constant expression
but an integer constant expression.