[Issue 23969] New: TLS in Windows cannot export/import TLS from DLLs

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=23969

          Issue ID: 23969
           Summary: TLS in Windows cannot export/import TLS from DLLs
           Product: D
           Version: D2
          Hardware: All
                OS: Windows
            Status: NEW
          Severity: normal
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: bugzilla at digitalmars.com

The following C code:

    __declspec(dllexport) __declspec(thread) int xxx = 3;

fails to compile with VC with:

    test.c(2): error C2492: 'xxx': data with thread storage duration may not
have dll interface

So it looks like we can't reasonably make that work for D on Windows. The
`export` will have to be ignored, because making it an error will make it
clumsy to use `export:` attributes.

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