/a.js(2,7): error TS2420: Class 'B' incorrectly implements interface 'Sig'.
  Index signature for type 'string' is missing in type 'B'.


==== /defs.d.ts (0 errors) ====
    interface Sig {
        [index: string]: string
    }
==== /a.js (1 errors) ====
    /** @implements {Sig} */
    class B  {
          ~
!!! error TS2420: Class 'B' incorrectly implements interface 'Sig'.
!!! error TS2420:   Index signature for type 'string' is missing in type 'B'.
    }
    