[llvm-bugs] [Bug 38091] New: No support for gcc "error" function attribute

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=38091

            Bug ID: 38091
           Summary: No support for gcc "error" function attribute
           Product: clang
           Version: unspecified
          Hardware: All
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Frontend
          Assignee: unassignedclangbugs at nondot.org
          Reporter: naruse at airemix.jp
                CC: llvm-bugs at lists.llvm.org

GCC 4.3 supports "error" function attribute, but clang doesn't support it.
https://gcc.gnu.org/onlinedocs/gcc-4.3.6/gcc/Function-Attributes.html#index-g_t_0040code_007berror_007d-function-attribute-2123

Below description is why I want clang to implement this.

This attribute is similar to "diagnose_if" and "enable_if" function attribute.
But "error" function attribute behaves different and useful.

```c
int rb_scan_args_bad_format(const char *fmt,int ofs)
__attribute__((diagnose_if(fmt, "bad scan arg format","error")));

int
main(int argc, char **argv) {
    if (0) {
        rb_scan_args_bad_format("foo", 3);
    }
}
```

clang show error above code though the function call is optimized out.
But gcc successfully compiles below code.

```c
int rb_scan_args_bad_format(const char *fmt,int ofs)
    __attribute__((error("bad scan arg format")));

int
main(int argc, char **argv) {
    if (0) {
        rb_scan_args_bad_format("foo", 3);
    }
}
```

For example Ruby uses this feature in rb_scan_args.
https://github.com/ruby/ruby/blob/v2_5_1/include/ruby/ruby.h
Its behavior is explained in
https://silverhammermba.github.io/emberb/c/#parsing-arguments
Recent Ruby uses macros to implement it in order to handle its argument at
compile time.
And it uses "error" function attribute if its format argument is invalid.
In this use case, "error" function attribute's ignored if optimized out
behavior is very useful.

This is why I want clang also implement "error" function attribute.

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