未使用功能的链接器错误：它们何时发生？

Assume a static library libfoo that depends on another static library libbar for some functionality. These and my application are written in D. If my application only uses libfoo directly, and only calls functions from libfoo that do not reference symbols from libbar, sometimes the program links successfully without passing libbar to the linker and other times it doesn't.Assume a static library libfoo that depends on