More C type errors by default for GCC 14

Florian Weimer
Fri May 12 20:57:46 GMT 2023

* Jason Merrill:

> On Fri, May 12, 2023 at 11:03 AM Florian Weimer <> wrote:
>> * Joseph Myers:
>> > On Wed, 10 May 2023, Eli Zaretskii via Gcc wrote:
>> >
>> >> 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.
>> >
>> > At block scope,
>> >
>> >   auto x = 1.5;
>> >
>> > declares x to have type double in C2x (C++-style auto), but type int in
>> > C89 (and is invalid for versions in between).  In this case, there is an
>> > incompatible semantic change between implicit int and C++-style auto.
>> > Giving an error before we make -std=gnu2x the default seems like a
>> > particularly good idea, to further alert anyone who has been ignoring the
>> > warnings about implicit int that semantics will change incompatibly.
>> Obviously makes sense to me.
> Agreed.  But we could safely continue to accept
>   static x = 42;
> or even
>   auto x = 42; // meaning of 'auto' changes, meaning of the declaration does not
> We might make -Wimplicit-int an error by default only if the
> initializer has a type other than 'int'.

Based on what I saw fixing Fedora, these cases are not very common.
Sure, sometimes common program such as valgrind have an instance
<>, but that's really an

Implicit int is common as the return type of main (especially in
autoconf tests), and due to a missing declaration list entry of an
old-style function definition.  The main case could be treated as an
exception.  The old-style function definition case is a common source
of bugs and therefore worth fixing.  The addition of unnamed function
parameters as an extension actually created a new class of bugs here
(a typo in the type name of a single unnamed parameter results in an
old-style function definition by accident).

>> > In cases where the standard requires a diagnostic, some are errors, some
>> > are pedwarns-by-default or unconditional pedwarns, some are
>> > pedwarns-if-pedantic - the choice depending on how suspicious the
>> > construct in question is and whether it corresponds to a meaningful
>> > extension (this is not making an automatic choice for every such situation
>> > in the standard, it's a case-by-case judgement by maintainers).  By now,
>> > the cases discussed in this thread are sufficiently suspicious -
>> > sufficiently likely to result in unintended execution at runtime (not, of
>> > course, reliably detected because programs with such dodgy code are very
>> > unlikely to have thorough automated tests covering all their code) - that
>> > is it in the interests of users for them to be errors by default (for C99
>> > and later modes, in the cases that were valid in C89).
>> Just to recap, those are controlled by
>> -Wimplicit-function-declaration, -Wimplicit-int, -Wint-conversion, and
>> -Wincompatible-pointer-types, roughly in increasing order of
>> compatibility impact with old sources.
> What would the impact be of making -Wint-conversion an error by
> default only if the types are different sizes?

>From a distribution perspective, it does not change anything because
we build everything on 64-bit anyway.  Unlike e.g. Fedora, Debian
doesn't require all builds to succeed before the new package can be
installed, but given that the primary targets are 64 bit, I don't
think a restricted -Wint-conversion error would be much of a
simplification.  The target-dependent nature of the warning is
probably more confusing.

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