Suboptimal warning formatting with `bool` type in C

Suboptimal warning formatting with `bool` type in C

[peter0x44]

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Recently, I was writing some code, and noticed some slightly strange 
warning formatting on a function taking a `bool` parameter

#include <stdbool.h>
void test(bool unused)
{
}

bruh.c: In function 'test':
bruh.c:2:16: warning: unused parameter 'unused' [-Wunused-parameter]
     2 | void test(bool unused)
       |                ^

Notice that there is only a ^ pointing at the first character of the 
indentifer

There is no ~~~~ underlining. Also, only the first "u" is colored purple

The same issue does not manifest for _Bool

bruh.c: In function 'test':
bruh.c:2:17: warning: unused parameter 'unused' [-Wunused-parameter]
     2 | void test(_Bool unused)
       |           ~~~~~~^~~~~~

I was wondering why, and after some further investigation, I found the 
reason

gcc's stdbool.h uses:

#define bool    _Bool

to provide the type

I investigated that myself with:

#define test_type int

void test(test_type unused)

{
}

and also reproduced the same thing

bruh.c: In function 'test':
bruh.c:3:21: warning: unused parameter 'unused' [-Wunused-parameter]
     3 | void test(test_type unused)
       |                     ^

typedef however, does not have this problem.

So, I guess I'm asking:

1)

Why is #define used instead of typedef? I can't imagine how this could 
possibly break any existing code.

Would it be acceptable to make stdbool.h do this instead?

2)

Is it possible to improve this diagnostic to cope with #define?

also, it's worth noting, clang has this same "problem" too. Both the 
compiler emits the suboptimal underlining in the diagnostic, and its 
stdbool.h uses #define for bool

https://clang.llvm.org/doxygen/stdbool_8h_source.html

https://gcc.gnu.org/git/?p=gcc.git;a=blob;f=gcc/ginclude/stdbool.h

Re: Suboptimal warning formatting with `bool` type in C

[Joseph Myers]

On Wed, 1 Nov 2023, peter0x44 via Gcc wrote:

> Why is #define used instead of typedef? I can't imagine how this could
> possibly break any existing code.

That's how stdbool.h is specified up to C17.  In C23, bool is a keyword 
instead.

-- 
Joseph S. Myers
joseph@codesourcery.com

Re: Suboptimal warning formatting with `bool` type in C

[peter0x44]

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On 2023-11-01 23:13, Joseph Myers wrote:

> On Wed, 1 Nov 2023, peter0x44 via Gcc wrote:
> 
>> Why is #define used instead of typedef? I can't imagine how this could
>> possibly break any existing code.
> 
> That's how stdbool.h is specified up to C17.  In C23, bool is a keyword
> instead.

I see, I didn't know it was specified that way. It seems quite strange 
that typedef wouldn't be used for this purpose.

I suppose perhaps it matters if you #undef bool and then use it to 
define your own type? Still, it seems very strange to do this.

Maybe it's something to offer as a GNU extension? Though, I'm leaning 
towards too trivial to be worth it, just for a (very minor) improvement 
to a diagnostic that can probably be handled in other ways.

Re: Suboptimal warning formatting with `bool` type in C

[David Brown]

On 02/11/2023 00:28, peter0x44 via Gcc wrote:
> On 2023-11-01 23:13, Joseph Myers wrote:
> 
>> On Wed, 1 Nov 2023, peter0x44 via Gcc wrote:
>>
>>> Why is #define used instead of typedef? I can't imagine how this could
>>> possibly break any existing code.
>>
>> That's how stdbool.h is specified up to C17.  In C23, bool is a keyword
>> instead.
> 
> I see, I didn't know it was specified that way. It seems quite strange 
> that typedef wouldn't be used for this purpose.
> 
> I suppose perhaps it matters if you #undef bool and then use it to 
> define your own type? Still, it seems very strange to do this.
> 

Yes, that is part of the reason.  The C standards mandate a number of 
things to be macros when it would seem that typedef's, functions, 
enumeration constants or other things would be "nicer" in some sense. 
Macros have two advantages, however - you can "#undef" them, and you can 
use "#ifdef" to test for them.  This makes them useful in several cases 
in the C standards, especially for changes that could break backwards 
compatibility.  Someone writing /new/ code would hopefully never make 
their own "bool" type, but there's plenty of old code around - if you 
ever need to include some pre-C99 headers with their own "bool" type and 
post-C99 headers using <stdbool.h>, within the same C file, then it's 
entirely possible that you'll be glad "bool" is a macro.

> Maybe it's something to offer as a GNU extension? Though, I'm leaning 
> towards too trivial to be worth it, just for a (very minor) improvement 
> to a diagnostic that can probably be handled in other ways.
> 

Speaking as someone with absolutely zero authority (I'm a GCC user, not 
a GCC developer), I strongly doubt that "bool" will be made a typedef as 
a GCC extension.

But if there are problems with the interaction between pre-processor 
macros and the formatting of diagnostic messages, then that is 
definitely something that you should file as a bug report and which can 
hopefully be fixed.

David