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From: Keith Thompson <kst-u@mib.org>
Newsgroups: comp.lang.c
Subject: Re: memcpy not memcpying?
Date: Mon, 04 Nov 2019 10:03:28 -0800
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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.

gcc and clang seem to agree.

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Working, but not speaking, for Philips Healthcare
void Void(void) { Void(); } /* The recursive call of the void */