Re: More C type errors by default for GCC 14

[Richard Biener <richard.guenther@gmail.com>]

> Am 10.05.2023 um 18:31 schrieb Eli Zaretskii via Gcc <gcc@gcc.gnu.org>:
> 
> 
>> 
>> Date: Wed, 10 May 2023 18:02:53 +0200
>> From: Jakub Jelinek <jakub@redhat.com>
>> Cc: gabravier@gmail.com, jwakely.gcc@gmail.com, fweimer@redhat.com,
>>        gcc@gcc.gnu.org, arsen@aarsen.me
>> 
>>> If some program is plainly invalid, not just because the criteria of
>>> validity have shifted, then yes, such a program should be rejected.
>> 
>> Many of the accepts-invalid cases are when something used to be valid in some
>> older standard and is not valid in a newer standard, often even changes
>> meaning completely in even newer standard.
>> Examples include e.g. the auto keyword, which means something completely
>> different in C++11 and later than what it meant in C++98, or say comma in
>> array reference in C++17 vs. C++20 vs. C++23 (a[1, 2] is the same as a[(1, 2)]
>> in C++17, got deprecated in C++20 and is ill-formed or changed meaning
>> in C++23 (multi-dimensional array operator).
>> Or any time something that wasn't a keyword in older standard version
>> and is a keyword in a newer standard.
>> alignas/alignof/nullptr/static_assert/thread_local in C++11 and C23,
>> char16_t/char32_t/constexpr/decltype/noexcept in C++11,
>> constinit/consteval in C++20,
>> bool/false/true/typeof_unqual in C23.
>> 
>> int bool = 1;
>> is completely valid C17 if one doesn't include <stdbool.h> header,
>> or
>> int static_assert = 2;
>> valid C17 if one doesn't include <assert.h>
>> etc.  These used to compile and will no any longer wheen using -std=c2x or
>> in a few years when -std=gnu23 becomes the default will not compile by
>> default, even when it used to be valid C17.
> 
> The examples you gave are the ones I could accept as "good reasons"
> for breaking backward compatibility.  That's because breaking that is
> unavoidable if GCC wants to support the newer standard.
> 
> That is not the case we are discussing, AFAIU.  Or at least no one has
> yet explained why accepting those old K&R programs will adversely
> affect the ability of GCC to compile C2x programs.

But we are discussing to reject K&R programs only when C99 or later standards are applied (those are applied by default)

Richard