[Issue 21211] New: `__FILE__.ptr` as default argument gives wrong value

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=21211

          Issue ID: 21211
           Summary: `__FILE__.ptr` as default argument gives wrong value
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: pro.mathias.lang at gmail.com

Using `__FILE__.ptr` will get const-folded, meaning it will always be the
caller's site showing up:

```
// a.d
import b;

void main ()
{
    foo();
}
```

```
// b.d
import core.stdc.stdio;

void foo(const(char)* file = __FILE__.ptr)
{
    printf("Called from %s\n", file);
}
```

Using the latest DMD (v2.093.1):
```
$ dmd -i -run a.d
Called from b.d
```

But why would one ever use `__FILE__.ptr`, might you ask ? Just use `__FILE__`
!
Well I would, but in my original code, `foo` is `extern(C++)`...

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