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Union type method parameters in base class accepts incompatible types in subclass overrides #19832
Description
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.
Activity
ahejlsberg commentedon Nov 8, 2017
See here for an explanation:
https://github.com/Microsoft/TypeScript/wiki/FAQ#why-are-function-parameters-bivariant
Also, see #18654.
asmundg commentedon Nov 8, 2017
Aha, I had missed the part where
--strictFunctionTypesdoes not apply to methods. I take it I should stay away from inheritance if I want contravariant parameter checking then.