f35 kdevelop 5.6.2 :: within project sting_view not found in std?

Hi! I have a really weird problem with kdevelop:
in the problems tab i get:
No type named 'string_view' in namespace 'std'

i made sure that in the project configuration -> Laguage support -> C/C++ parser
i have c++ profile to c++17 and Compiler for path = GCC

Any idea why string_view could not be found in std?

Thanks a lot!
Adrian