Union type method parameters in base class accepts incompatible types in subclass overrides

[asmundg]

Using union types in base class method declarations allows override functions that only declare one of the union member types. This produces code that crashes at runtime, even with --strict.

TypeScript Version: 2.7.0-dev.20171108

Code

abstract class Base {
    abstract foo(arg: string | number): boolean
}

class C extends Base {
    foo(arg: string): boolean {
        return arg.startsWith("a")
    };
}

const a: Base = new C()
a.foo(1)

Expected behavior:

Should not compile.

Actual behavior:

Compiles and crashes at runtime.

[ahejlsberg]

See here for an explanation:

https://github.com/Microsoft/TypeScript/wiki/FAQ#why-are-function-parameters-bivariant

Also, see #18654.

[asmundg]

Aha, I had missed the part where --strictFunctionTypes does not apply to methods. I take it I should stay away from inheritance if I want contravariant parameter checking then.