For this inspection to work, a superclass needs to be in project source files and the project needs to use the IntelliJ IDEA build system.
Example:
class MyInheritor implements A {} // B suggested on the A reference
interface A {}
abstract class B implements A {}
abstract class C1 extends B {}
abstract class C2 extends B {}
abstract class C3 extends B {}
abstract class C4 extends B {}
abstract class C5 extends B {}
By default, this inspection doesn't highlight issues in the editor but only provides a quick-fix.
New in 2017.2