Support interfaces having multiple base interfaces

[CountBleck]

Changes proposed in this pull request:
⯈Adding support for an interface extending multiple interfaces.

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[CountBleck]

@dcodeIO How does it look?

[CountBleck]

Whoops, needed to rebase. It should work now.

[HerrCai0907]

Could you explain more about the target behavior of extended interfaces and how to handle the conflict between different base interfaces.
Maybe you need to add more test cases for those.

[CountBleck]

Could you explain more about the target behavior of extended interfaces and how to handle the conflict between different base interfaces.
Maybe you need to add more test cases for those.

Ah, I didn't see this. From my understanding, I don't think there should be any conflict, because the implementer needs to satisfy all of the requirements imposed by each interface, whereas the interface has no requirements imposed on it by the implementer, asides from a case being added to each of the interface's override (virtual) stubs.

I could of course be very wrong about this, but I don't know what else should be tested.

[HerrCai0907]

interface A {
  f(): void;
}

interface B {
  f(): i32;
}

interface C extends A, B {}

class D implements C {
  f(): void {}
}

For this case, TS will diagnose Interface 'C' cannot simultaneously extend types 'A' and 'B'. Named property 'f' of types 'A' and 'B' are not identical.ts(2320) but AS will not.

[CountBleck]

By the way,

interface A {
    m: Object | null
}

interface B {
    m: Object
}

interface X extends A, B {
    // m: Object
}

isn't valid in TypeScript, but uncommenting that line will fix the error.

[HerrCai0907]

isn't valid in TypeScript, but uncommenting that line will fix the error.

So at least we need to know what is correct and what is wrong firstly to avoid huge mismatch with TS.
Normally type system is very complex module which has lots of corner cases.
Maybe we can start from the most strict mode and forbidden each same name declaration and release limitation one by one to avoid publishing break change over and over.

[CountBleck]

I believe the error is only emitted if the property isn't explicitly defined on the interface. Maybe some more digging is warranted though.

Also, another thing about the "simultaneously extend" error: for each conflicting property, it chooses the first interface with that property to be the first interface mentioned in each error (for that property), and the second interface is each of the remaining interfaces.

I can't explain it that clearly, so here's an example. x and y are the conflicting properties. For x, B comes first, so there are errors for B and C, as well as B and D. For y, C comes first, so there are errors for C and D, as well as C and E.

[HerrCai0907]

So this PR still missing combination check when inheriting from difference base interfaces, right?

[CountBleck]

Both of those things I described are part of that diagnostic, which I haven't implemented yet.

[lebrunel]

FWIW we're actually running this PR in our fork of assemblyscript, and we added a small number of extra diagnostics messages to our custom compiler for different interface scenarios. I can share our test cases if that would be helpful?

[CountBleck]

@lebrunel that would be great, thanks!

[lebrunel]

The following is by no means exhaustive (we don't have a test for the simultaneous extending case mentioned above), but hopefully these are in someway helpful. At the very least you'll be able to write some tests that fail :P

// implementing a different method signature should fail
// Types of property `m` are incompatible.
interface A {
  m(): void;
}

class B implements A {
  m(n: u8): void {}
}

// implementing a different method signature from nested interfaces should fail
// Types of property `m1` are incompatible.
interface A {
  m1(): void;
}

interface B extends A {
  m2(): void;
}

interface C extends B {
  m3(): void;
}

class D implements C {
  m1(n: u8): void {}
  m2(): void {}
  m3(): void {}
}

// implementing a different method signature with compatible types should pass
interface A {
  m1(): A;
}

interface B extends A {
  m2(): B;
}

class C implements A {
  m1(): C { return this };
}

class D implements B {
  m1(): D { return this };
  m2(): B { return this };
}

// extending a different method signature should fail
// Types of property `m` are incompatible.
interface A {
  m(): void;
}

interface B extends A {
  m(n: u8): void;
}

// extending a different method signature from nested interfaces should fail
// Types of property `m1` are incompatible.
interface A {
  m1(): void;
}

interface B extends A {
  m2(): void;
}

interface C extends B {
  m1(n: u8): void;
}

// implementing a different field type with compatible types should pass
interface A {
  a: A;
  b: A | null;
}

class B extends Jig implements A {
  a: B;
  b: B | null;

  constructor(a: B, b: B) {
    super()
    this.a = a
    this.b = b
  }
}