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From: Tim Rentsch <tr.17687@z991.linuxsc.com>
Newsgroups: comp.lang.c
Subject: Re: Bart's Language
Date: Sun, 27 Apr 2025 11:53:05 -0700
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Keith Thompson <Keith.S.Thompson+u@gmail.com> writes:

> antispam@fricas.org (Waldek Hebisch) writes:
> [...]
>
>> Well, it is rather easy to see if variable is used within its
>> own initialization, so practically it is minor gap.  Of course,
>> there is problem with C standard:  IIUC depending on rest of
>> the code declarations as above are merely undefined behaviour
>> or even produce unspecified value.  So C compiler is
>> forbidden to stop compilation are report compile time error.
>
> Valid responses to undefined behavior include "terminating a translation
> or execution (with the issuance of a diagnostic message)".

That's true, but doing so is allowed only if the circumstances
of undefined behavior have occurred.  In the case of compiling
a declaration such as

    int a = a;

no undefined behavior has as yet occurred.

> In other
> words, if a compiler is able to prove that a program has undefined
> behavior (that will occur on each execution), it can reject it at
> compile time.

The program can be rejected, but not because of the rule about
terminating a translation.  The program can be rejected because
the program is not strictly conforming, and implementations are
not required to accept programs that are not strictly conforming.