Bug 20684 – "Error: unknown" with some bad nested struct/template declarations

/////////////// test.d /////////////// struct RD { struct BB { R!RD r; } Undeclared undeclared; struct VV { vector!int v; } struct VVV { vector!VV vv; } } struct R(T) { static if (is(typeof(*p = *p))) { } T* p; } struct vector(T) { Array!T array; } struct Array(T) { struct Payload { T[] _payload; } ~this() { foreach (e; _data._payload) {} } Payload _data; this()() { } } ////////////////////////////////////// Compiler says: $ dmd test.d Error: unknown, please file report on issues.dlang.org test.d(17): Error: template instance test.vector!(VV) error instantiating

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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