[Issue 21108] New: Wrong mangling for extern(C++, std) (identifier namespace)

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=21108

          Issue ID: 21108
           Summary: Wrong mangling for extern(C++, std) (identifier
                    namespace)
           Product: D
           Version: D2
          Hardware: x86_64
                OS: Linux
            Status: NEW
          Keywords: C++
          Severity: normal
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: pro.mathias.lang at gmail.com

Taken from: https://issues.dlang.org/show_bug.cgi?id=16479#c11

```
extern (C++, std)
struct pair(T1, T2) {}

extern (C++)
void func_20413(pair!(int, float), pair!(float, int));
```

actual:   _Z10func_20413St4pairIifEStS_IfiE
expected: _Z10func_20413St4pairIifES_IfiE

A C++ string namespace `extern (C++, "std") struct pair(T1, T2) {}` works as
expected (testcase from https://issues.dlang.org/show_bug.cgi?id=20413).

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