WebStorm - Argument type {providedIn: "root"} is not assignable to parameter type {providedIn: Type<any> | "root" | null} & InjectableProvider
Believe me or not, I closed and opened my project in Webstorm and the error just disappeared
Thx @yurzui and @AdrianFâciu for the support
UPDATE
It turns out it was a confirmed bug in Webstorm which was later corrected in v2018.1.5 and published Sunday 17th August 2018. I have updated my editor to this version and didn't face the problem anymore
Webstorm issue tracker: https://youtrack.jetbrains.com/issue/WEB-32634
v2018.1.5 release notes: https://confluence.jetbrains.com/display/WI/WebStorm+181.5281.31+Release+Notes