CLion incorporates two language engines: the built-in one and the Clangd-based one.
The Clangd-based engine is used to detect warnings and errors to be shown in the editor, while quick-fixes and other code insight features (i.e. code highlighting, refactorings, completion, navigation) remain provided by the CLion's own code engine. For configuration details, see the Clangd reference page.
The built-in language engine in CLion supports the C++ standards up to C++14 and partially C++17. Tables below show the lists of each standard's features marked as supported or currently unsupported in CLion.