[Issue 20684] "Error: unknown" with some bad nested struct/template declarations

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=20684

--- Comment #1 from Vladimir Panteleev <dlang-bugzilla at thecybershadow.net> ---
Compilation succeeds in DMD 2.085 but not newer.

Bisecting shows that the error message begins manifesting when it was
introduced ( https://github.com/dlang/dmd/pull/9425 ), however debug versions
of DMD have been failing with an assertion failure before that change. The
assertion failures began after https://github.com/dlang/dmd/pull/6690 .

See also issue 20753

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