1. Enable the ClangCodeModel plugin and restart Creator.
2. In "Tools" -> "Options..." -> "C++" -> "Code Model", enable the Clang Code Model.
3. Create a new Plain C++ Project.
4. Make the include invalid by changing the desired filename.
   Clang will detect the error but only underline the "<":
   

When there's an error in the code, the whole line should be underlined. With Creator's built-in code model, this works correctly.