[Issue 19260] extern(C++) `T* const` mangling

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=19260

--- Comment #8 from Manu <turkeyman at gmail.com> ---
We could give the argument a @MangleHeadConst attribute or something that the
mangler recognises...?

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