Scala editor does not show all errors

(IDEA + Scala plugins latest versions)
When I edit a Scala file it shows some types of errors, such as unmatched parens / curly braces.
But it does not highlight invalid identifiers at all! So for example, I can type the following:

a.b(c)

and it will not color any part of it red, even though I have no "a", "b" and "c" in scope at all.

I have the default settings in Editor / Colors & Fonts.
Power Save Mode is turned off.

How can I turn on the highlighting of invalid identifiers / function names?