C23 `typeof()` cannot be used in a `union` or `struct` with a compound literal with a non-constant initializer

[g-berthiaume]

Hi,

I just encountered what seems to be the same problem as #25689, but with C23 typeof() instead of the __typeof__() intrinsics.
Let me know if this issue is redundant.

Here's a reproducible example:

How to reproduce:

#include <stdlib.h>

int main(void)
{
    int x = 3;
    struct { typeof((int){x}) y; };
    return 0;
}

Compile with

clang -std=c23 .\main.c -o main
.\main.c:6:26: error: initializer element is not a compile-time constant
    6 |     struct { typeof((int){x}) y; };
      |                          ^~~
1 error generated.

The error has been reproduced on Windows:

clang version 20.1.8
Target: x86_64-pc-windows-msvc
Thread model: posix

The error has been reproduced on Linux (Ubuntu WSL):

Ubuntu clang version 18.1.8 (++20240731024944+3b5b5c1ec4a3-1~exp1~20240731145000.144)
Target: x86_64-pc-linux-gnu
Thread model: posix

Motivation

While a minimal reproducible example is nice, here's an example to motivate the PR:
The creation of a UNION_CAST(int, my_var) macro for safe C type punning.

// :: UNION_CAST
#define UNION_CAST(NEW_TYPE, VAR)        \
        (((union {                       \
                 typeof(VAR) before;    \
                 NEW_TYPE after;         \
         }){.before = (VAR)})            \
             .after)

Thanks for your work!

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: G. Berthiaume (g-berthiaume)

Hi,

I just encountered what seems to be the same problem as #25689, but with C23 typeof() instead of the __typeof__() intrinsics.

Here's a reproducible example:
Let me know if this issue is redundant.

How to reproduce:

#include <stdlib.h>

int main(void)
{
    int x = 3;
    struct { typeof((int){x}) y; };
    return 0;
}

Compile with

clang -std=c23 main.c -o main
.\main.c:8:25: error: initializer element is not a compile-time constant
    8 |         __typeof__((int){x}) y;
      |                         ^~~
1 error generated.

The error has been reproduced on Windows:

clang version 20.1.8
Target: x86_64-pc-windows-msvc
Thread model: posix

The error has been reproduced on Linux (Ubuntu WSL):

Ubuntu clang version 18.1.8 (++20240731024944+3b5b5c1ec4a3-1~exp1~20240731145000.144)
Target: x86_64-pc-linux-gnu
Thread model: posix

Motivation

While a minimal reproducible example is nice, here's an example to motivate the PR:
The creation of a UNION_CAST(int, my_var) macro for safe C type punning.

// :: UNION_CAST
#define UNION_CAST(NEW_TYPE, VAR)        \
        (((union {                       \
                 typeof(VAR) before;    \
                 NEW_TYPE after;         \
         }){.before = (VAR)})            \
             .after)

Thanks for your work!

[g-berthiaume]

@EugeneZelenko I think this is fixed in trunk

https://godbolt.org/z/TT6Gn4jMj

Would you prefer that I close this issue or keep it open until 21.0.0 is released?

[EugeneZelenko]

@g-berthiaume: I think this depends on should fix be backported to 21.x or not.

[Sirraide]

Let me know if this issue is redundant.

In this case it is because __typeof__ is just an alias for typeof.

I think this depends on should fix be backported to 21.x or not.

CC @AaronBallman Do we want to backport this—assuming we know what fixed it?

Also, do we usually use issues to track that? Either way, I think leaving #25689 open should be enough in that case.

[frederick-vs-ja]

@EugeneZelenko @Sirraide I think #143852 fixed this issue (and that PR was already merged in LLVM 21).

Closing as duplicate #87867 instead as that issue was said to be fixed.