This class serves as a type in the testing namespace to be passed to an unqualified call to PrintTo. By supplying bslim_Gtestutil_TestingStreamHolder(&stream), which is implicitly convertible to bsl::ostream *, we supply an argument in the testing namespace in the UniversalPrint call within the bsl::PrintTo(const optional<TYPE>&, ...) call below, which will affect ADL to draw in UniversalPrint declarations from the testing namespace into consideration. For detailed discussion, see IMPLEMENTATION NOTE in the implementation file.
See bslim_gtestutil