TypeScript Version:

Version 1.9.0-dev.20160617-1.0

Code

interface Foo { prop?: string [key: string]: string }

{ "compilerOptions": { "strictNullChecks": true } }

Expected behavior: No error. Wouldn't the fact that it's an index signature implicitly signal that it could also be undefined?

Actual behavior:

index.ts(2,3): error TS2411: Property 'prop' of type 'string | undefined' is not assignable to string index type 'string'. 

Edit: On the other side of the coin with this usage:

interface Foo { [key: string]: string } const foo: Foo = {} const bar = foo['test']

Should bar be type of string | undefined?