[Issue 1830] New: duplicated constants + extern(Windows) = Link error: Previous Definition Different

[d-bugmail at puremagic.com]

http://d.puremagic.com/issues/show_bug.cgi?id=1830

           Summary: duplicated constants + extern(Windows) =  Link error:
                    Previous Definition Different
           Product: D
           Version: 1.025
          Platform: PC
        OS/Version: Windows
            Status: NEW
          Keywords: link-failure
          Severity: normal
          Priority: P2
         Component: DMD
        AssignedTo: bugzilla at digitalmars.com
        ReportedBy: wbaxter at gmail.com


If two modules define the same constants it's usually OK.  But if those
constants are extern(windows) then they will create linker errors.

---link_main.d---
module link_main;
import link_a;
import link_b;
void main()
{
}

---link_a.d---
module link_a;

extern(Windows):
const uint FOO = cast(uint)-1;


---link_b.d---
module link_b;

extern(Windows):
const uint FOO = cast(uint)-1;

---

Build with 
  dmd link_main link_a link_b

And you get:
"""
f:\usr\pkg\d\dmd\bin\..\..\dm\bin\link.exe
link_main+link_a+link_b,,,user32+kernel32/noi;
OPTLINK (R) for Win32  Release 7.50B1
Copyright (C) Digital Mars 1989 - 2001  All Rights Reserved

link_b.obj(link_b)  Offset 000E0H Record Type 0091
 Error 1: Previous Definition Different : _FOO
--- errorlevel 1
"""

If you remove the extern(Windows) lines, it links fine.

This is causing trouble when linking apps that import more than one Windows API
wrapper since most of them stick an extern(Windows): at the top of the file,
which ends up covering all the constants as well as functions.   I'm not even
sure if extern(Windows) has meaning for constants like that, but it certainly
appears to influence how the linker handles them.


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