src/a.ts(2,10): error TS2345: Argument of type '{ foo: (v: string) => boolean; }' is not assignable to parameter of type '{ foo: (val: {}) => boolean; }'.
  Types of property 'foo' are incompatible.
    Type '(v: string) => boolean' is not assignable to type '(val: {}) => boolean'.
      Types of parameters 'v' and 'val' are incompatible.
        Type '{}' is not assignable to type 'string'.