[Issue 20652] extern(C++) doesn't seem to mangle the types in core.simd right

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=20652

--- Comment #4 from Iain Buclaw <ibuclaw at gdcproject.org> ---
(In reply to Iain Buclaw from comment #2)
> And on what ABI?  MSVC++ or Itanium?
I've just realised that there's still this in cppmangle.d

---
version (none)
{
    buf.writestring("Dv");
    buf.print((cast(TypeSArray *)t.basetype).dim.toInteger()); // -- Gnu ABI
v.4
    buf.writeByte('_');
}
else
    buf.writestring("U8__vector"); //-- Gnu ABI v.3
---

I thought I dealt with this after introducing target.cpp.typeMangle().  A pull
has been raised to enable the V4 ABI path.  The V4 ABI first appeared in g++
4.5, and became the default in g++ 5.

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