| Issue |
179403
|
| Summary |
Clang rejects valid C11 code: invalid operands to binary _expression_ for _Atomic types returned from functions
|
| Labels |
clang
|
| Assignees |
|
| Reporter |
jupiter-zzp
|
### Description:
Clang fails to compile valid C11 code that compares two _Atomic objects using the != operator when one operand is a function return value. The same comparison works correctly when both operands are variables (not function returns). GCC accepts this code without errors.
### Reproducer:
[Live demo on Godbolt Compiler Explorer](https://godbolt.org/z/o6zPo3Ko5)
```
#include <stdint.h>
#include <stdatomic.h>
int test(atomic_size_t a, atomic_size_t b) {
return a != b;
}
atomic_size_t foo(atomic_size_t a) {
return a;
}
int test_OK(atomic_size_t a, atomic_size_t b) {
atomic_size_t c = foo(a);
return c != b;
}
int test_error(atomic_size_t a, atomic_size_t b) {
return foo(a) != b;
}
```
### Output:
```
<source>:18:19: error: invalid operands to binary _expression_ ('atomic_size_t' (aka '_Atomic(size_t)') and 'atomic_size_t')
18 | return foo(a) != b;
| ~~~~~~ ^ ~
1 error generated.
Compiler returned: 1
```
### Expected Behavior:
The code should compile successfully, as atomic_size_t (aka _Atomic(size_t)) objects should be comparable using the != operator regardless of whether they are variables or function return values. This is consistent with C11 standard section 6.5.9 (Equality operators) which allows comparison of atomic types.
### Environment:
Clang version: 21.1.0 (also reproduced on other version)
Target: riscv32-unknown-unknown-elf (also reproduced on arm and x86)
GCC comparison: GCC 15.2.0 accepts this code
### Additional Context:
This appears to be a semantic analysis issue where Clang fails to properly handle value categories or type decay for atomic types when they are returned from functions. The direct comparison a != b works because both are lvalues of atomic type, but foo(a) != b fails because foo(a) is an rvalue (function return), suggesting Clang is not applying the same type conversion rules for atomic types in this context.
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