[Bug c++/116954] [13/14/15 Regression] format attribute is being lost for function template with a depedent type argument and decl merging

[pinskia at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116954

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2024-10-03
     Ever confirmed|0                           |1
      Known to work|                            |12.1.0, 12.4.0, 7.1.0,
                   |                            |9.1.0
            Summary|-Wsuggest-attribute=format  |[13/14/15 Regression]
                   |false positive for function |format attribute is being
                   |template                    |lost for function template
                   |                            |with a depedent type
                   |                            |argument and decl merging
           Keywords|                            |diagnostic
             Status|UNCONFIRMED                 |NEW
      Known to fail|13.2.0, 13.3.0, 13.3.1,     |3.4.6
                   |14.1.0, 14.2.0              |

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Actually the problem is not with `Wsuggest-attribute=format` but rather the C++
front-end losing the format attribute when it comes to dependent array types
and merging decls.

Take:
```
#ifndef WORKS
template<int N>
int fn(char (&buf)[N], const char fmt[], ...)
  __attribute__ ((__format__ (__printf__, 2, 3)));
#endif

template<int  N>
__attribute__ ((__format__ (__printf__, 2, 3)))
int fn(char (&)[N], const char [], ...)
{ return 0; }

int main()
{
  char buf[20];
  return fn(buf, "%s", 42); /* { dg-warning "Wformat" } */
}
```

This used to work in some versions of GCC but no longer works.