Dart chooses widest possible interpretation of type parameter when inferring from function arguments

[roblframpton]

Consider this function:

T myFunc<T>(T input1, T input2) {
    return input1;
}

If I call this function like so:

final x = myFunc(123, 234);

then T is int, but if I call it like this:

final x = myFunc(123, "hello");

then T is Object. I can see why - it's the nearest possible supertype of both int and String. Unfortunately for my case, this is not what I wanted. I was hoping to restrict my fuction so that if the first argument was an int, the second argument is also an int.

I can see that Dart's behaviour here is not in any way wrong, but I'm curious to know why this choice was made, and whether there is a way to change the behaviour to get the effect I wanted.

Just by way of comparison, TypeScript does behave the way I had hoped:

function myFunc<T>(arg1: T, arg2: T): T {
  return arg1;
}

const z = myFunc(1 as number, ""); // Error: Argument of type 'string' is not assignable to parameter of type 'number'

class A {f1(){};}
class B {f2(){};}

const a = myFunc(new A(), new B()); // Error: Argument of type 'B' is not assignable to parameter of type 'A'.

What is Dart's justification for its behaviour, and are there situations where this is advantageous? Can I force it to behave otherwise?

[Mike278]

Looks similar to #3156

I was hoping to restrict my fuction so that if the first argument was an int, the second argument is also an int.
Can I force it to behave otherwise?

If you want the two arguments to be exactly the same type, you'll have to emulate invariance as described in #3156 (comment).

But depending on your use case, adding a subclass constraint might also be satisfactory:

A myFunc<A, B extends A>(A input1, B input2) {
    return input1;
}

Now when you write

final x = myFunc(123, "hello");

you get

Couldn't infer type parameter 'B'.

Tried to infer 'String' for 'B' which doesn't work:
  Type parameter 'B' is declared to extend 'A' producing 'int'.
The type 'String' was inferred from:
  Parameter 'input2' declared as     'B'
                     but argument is 'String'.

Consider passing explicit type argument(s) to the generic.

[roblframpton]

Yes, that's exactly what I hoped for. Thank you!