a.ts(1,11): error TS2430: Interface 'Merged' incorrectly extends interface 'Base'.
  Types of property 'onerror' are incompatible.
    Type '(ev: number) => void' is not assignable to type '(ev: string) => void'.
      Types of parameters 'ev' and 'ev' are incompatible.
        Type 'string' is not assignable to type 'number'.
b.ts(1,11): error TS2430: Interface 'Merged' incorrectly extends interface 'Base'.
  Types of property 'onerror' are incompatible.
    Type '(ev: number) => void' is not assignable to type '(ev: string) => void'.
      Types of parameters 'ev' and 'ev' are incompatible.
        Type 'string' is not assignable to type 'number'.
b.ts(43,5): error TS2411: Property 'bBad' of type 'string' is not assignable to 'string' index type 'number | ((ev: number) => void)'.
c.ts(43,5): error TS2411: Property 'cBad' of type 'string' is not assignable to 'string' index type 'number | ((ev: number) => void)'.
d.ts(43,5): error TS2411: Property 'dBad' of type 'string' is not assignable to 'string' index type 'number | ((ev: number) => void)'.
