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

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.

See here for an explanation:

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

Also, see #18654.

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.

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Union type method parameters in base class accepts incompatible types in subclass overrides #19832

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Union type method parameters in base class accepts incompatible types in subclass overrides #19832

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ahejlsberg commented on Nov 8, 2017

asmundg commented on Nov 8, 2017

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