[Issue 21178] It is not explained what is "unknown"

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=21178

kinke <kinke at gmx.net> changed:

           What    |Removed                     |Added
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                 CC|                            |kinke at gmx.net

--- Comment #1 from kinke <kinke at gmx.net> ---
That's very subjective - I find it crystal-clear, especially with the 'opaque'
hint - it's just like an opaque `struct S;` declaration. Sometimes, a full
declaration isn't needed, e.g.:

extern(C++):
enum E : int;
void foo(E);

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