unresolved external symbol internal. (Smaller test case)

main.d

// dmd -m64 -lib -of="lib.lib" -debug -g -w  -I="./" texteditor.d
// dmd -m64 -of="app.exe" -debug -g -w -I="./" lib.lib main.d
import texteditor;
void main() {}

texteditor.d

module texteditor;

class EditorTextModel
{
	TextEditor* editor;
	ChunkedRange!char opSlice(size_t from, size_t to) { return editor.textData[from..to].toChunkedRange; }
	void replaceSelection(const(char)[] str) { editor.replaceSelection(str); }
}

struct TextEditor
{
	private PieceTable textData;
	mixin ReadHelpers!();
	mixin WriteHelpers!();
}

mixin template ReadHelpers()
{
	import std.datetime : MonoTime;
}

mixin template WriteHelpers()
{
	void replaceSelection(const(char)[] str) {}
}

private struct Piece { size_t bufferOffset; }
struct PieceRestoreRange {}

private struct PieceStorage
{
	Piece* sentinel = new Piece;
	PieceRestoreRange insertAt(Piece* newPiece, size_t insertPos)
	{
		return PieceRestoreRange();
	}
}

struct PieceTable
{
	PieceStorage pieces;
	Range!char opSlice(size_t x, size_t y) { return Range!char(); }
	size_t length() @property { return 0; }
}

private struct Range(T)
{
	ChunkedRange!char toChunkedRange() { return ChunkedRange!char(); }
}

struct ChunkedRange(T) { size_t itemsLeft; }