counted_by cannot be used with C23 attribute syntax

[tavianator]

The docs claim that counted_by has a C23 attribute syntax equivalent of [[clang::counted_by(...)]]. However:

tavianator@graphene$ cat foo.c
struct foo {
        int n;
        int a[] [[clang::counted_by(n)]];
};

struct bar {
        int n;
        int a[] __attribute__((counted_by(n)));
};
tavianator@graphene$ clang -fsyntax-only foo.c
foo.c:3:12: error: 'counted_by' attribute cannot be applied to types
    3 |         int a[] [[clang::counted_by(n)]];
      |                   ^
1 error generated.
tavianator@graphene$ clang -v
clang version 20.1.6
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin
Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-pc-linux-gnu/15.1.1
Found candidate GCC installation: /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/15.1.1
Selected GCC installation: /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/15.1.1
Candidate multilib: .;@m64
Candidate multilib: 32;@m32
Selected multilib: .;@m64

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Tavian Barnes (tavianator)

[The docs](https://clang.llvm.org/docs/AttributeReference.html#counted-by-counted-by-or-null-sized-by-sized-by-or-null) claim that `counted_by` has a C23 attribute syntax equivalent of `[[clang::counted_by(...)]]`. However:

tavianator@<!-- -->graphene$ cat foo.c
struct foo {
        int n;
        int a[] [[clang::counted_by(n)]];
};

struct bar {
        int n;
        int a[] __attribute__((counted_by(n)));
};
tavianator@<!-- -->graphene$ clang -fsyntax-only foo.c
foo.c:3:12: error: 'counted_by' attribute cannot be applied to types
    3 |         int a[] [[clang::counted_by(n)]];
      |                   ^
1 error generated.
tavianator@<!-- -->graphene$ clang -v
clang version 20.1.6
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin
Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-pc-linux-gnu/15.1.1
Found candidate GCC installation: /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/15.1.1
Selected GCC installation: /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/15.1.1
Candidate multilib: .;@<!-- -->m64
Candidate multilib: 32;@<!-- -->m32
Selected multilib: .;@<!-- -->m64

[EugeneZelenko]

GCC does not accept code either: https://godbolt.org/z/9MTnobq9c

[tavianator]

Indeed! But they do accept code like this:

struct foo {
	int n;
	[[clang::counted_by(n)]] int a[];
};

So maybe this can be closed, but it might be nice to document this

[shafik]

CC @rapidsna @AaronBallman

[AaronBallman]

The two spellings are equivalent in terms of what they do, but their syntactic positions are not equivalent, so this is expected behavior thanks to the array declarator being there, which can be a surprise.

int a [[attr]]; // Applies attr to a
int b[] [[attr]]; // Applies attr to the type int[]
int *c [[attr]]; // Applies attr to c
int (*d)() [[attr]]; // Applies attr to the type int(*)()

but you can write either of these and they're equivalent:

[[attr]] int a[];
int a [[attr]][];

This isn't specific to the counted_by attribute, it's with all attributes. The C++ behavior is documented by:
https://en.cppreference.com/w/cpp/language/declarations.html
And C is documented by:
https://en.cppreference.com/w/c/language/declarations.html

(and likely plenty of other resources online.) We don't generally document core language behavior (as opposed to things like compiler extensions where we're the authority).

Closing the issue as by design, but if you have further comments, feel free to reopen!