==== tests/cases/compiler/indexSignatureWithInitializer.ts (4 errors) ====
    interface I {
        [x = '']: string;
         ~
!!! An index signature parameter cannot have an initializer.
         ~~~~~~
!!! A parameter initializer is only allowed in a function or constructor implementation.
    }
    
    class C {
        [x = 0]: string
         ~
!!! An index signature parameter cannot have an initializer.
         ~~~~~
!!! A parameter initializer is only allowed in a function or constructor implementation.
    }