[Issue 21299] New: [LINK] undefined reference to dmd.root.stringtable.StringValue!(Type).StringValue.lstring()

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=21299

          Issue ID: 21299
           Summary: [LINK] undefined reference to
                    dmd.root.stringtable.StringValue!(Type).StringValue.ls
                    tring()
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: blocker
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: ibuclaw at gdcproject.org

Reduced from link error found in regression from Issue 21294 (which caused a
refactoring change in dmd to be reverted).

It may not be *the* cause of the Issue 21294, but it's certainly a major issue
that needs to be addressed, as this is preventing code in the dmd front-end
from being moved around.

Test script:
```
dmd -c dmd/mtype.d 
dmd -c dmd/func.d 
dmd -c dmd/root/stringtable.d 
dmd -main mtype.o func.o stringtable.o
```

Test sources
```
// dmd/func.d
module dmd.func;
import dmd.mtype;
import dmd.root.stringtable;
class FuncDeclaration {
    StringTable!Type stringtable;
}

// dmd/mtype.d
module dmd.mtype;
import dmd.func;
import dmd.root.stringtable;
class Type {
    StringTable!Type stringtable;
}

// dmd/root/stringtable.d
module dmd.root.stringtable;
struct StringValue(T)
{
    char* lstring()
    {
        return cast(char*)&this;
    }
}

struct StringTable(T)
{
    StringValue!T* insert()
    {
        allocValue;
        return getValue;
    }

    uint allocValue()
    {
        StringValue!(T) sv;
        sv.lstring[0] = 0;
        return 0;
    }

    StringValue!T* getValue()
    {
        return cast(StringValue!T*)&this;
    }
}
```

Results in the link error:
```
/usr/bin/ld: mtype.o: in function
`_D3dmd4root11stringtable__T11StringTableTCQBo5mtype4TypeZQBe10allocValueMFNaNbNiNfZk':
dmd/mtype.d:(.text._D3dmd4root11stringtable__T11StringTableTCQBo5mtype4TypeZQBe10allocValueMFNaNbNiNfZk[_D3dmd4root11stringtable__T11StringTableTCQBo5mtype4TypeZQBe10allocValueMFNaNbNiNfZk]+0x13):
undefined reference to
`_D3dmd4root11stringtable__T11StringValueTCQBo5mtype4TypeZQBe7lstringMFNaNbNiNfZPa'
collect2: error: ld returned 1 exit status
Error: linker exited with status 1
```

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