ExtensionProtocolforStruct

编译 ExtensionProtocolforStruct 项目时，链接器崩溃并显示以下错误消息： Undefined symbols for architecture x86_64: "__TFV9submodule10testStructg3fooSS", referenced from: __TTWV9submodule10testStruct26ExtensionProtocolforStruct12testProtocolS1_FS2_g3fooSS in ProtocolExtension.o ld: symbol(s) not found for architecture x86_64 clang: error: linker command failed with exit code 1 (use -v to see invocation) 设置是：