[Issue 18955] extern(C++) default struct mangling is overridden when interacting with a `cppmangle = class` template

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=18955

--- Comment #7 from Mike Franklin <slavo5150 at yahoo.com> ---
Attempted fix:  https://github.com/dlang/dmd/pull/8631

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