[Issue 20413] New: C++ mangling bug with templates & the std namespace

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=20413

          Issue ID: 20413
           Summary: C++ mangling bug with templates & the std namespace
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: sahmi.soulaimane at gmail.com

Test case:

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

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

pragma(msg, fun.mangleof);
```

output: _Z3funSt4pairIifENS_IfiEE
expected: _Z3funSt4pairIifES_IfiE

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