Allow visibility on constructors

[AbubakerB]

Fixes #2341.

Changes:
1 - Allows private, protected and public accessibilities on constructors.
2 - Disallows classes with private constructors from being extended.
3 - Now throws an error on constructor overloads with different access modifiers (than the constructors implementation).

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[RyanCavanaugh]

For this to work properly, the visibility needs to be part of the construct signature rather than being a declaration-based check that happens during signature resolution. Otherwise it's going to be too difficult to enforce assignability constraints based on constructor visibility.

A relevant testcase to add would be:

class Foo {
  private constructor() { }
}
class Bar {
  protected constructor() { }
}
let x: new() => any;
x = Foo; // Should be an error
x = Bar; // Should be an error

[AbubakerB]

@RyanCavanaugh SignaturesRelatedTo now checks visibility of constructor.

[AbubakerB]

Should i close this off? or is this in the pipeline?

[DanielRosenwasser]

Can you wrap these return statements with curlies and place them on their own line?

if (!constructor || !node.expression) {
    return;
}

[AbubakerB]

Done

[DanielRosenwasser]

The declaration emitter will need to be amended - add // @declaration: true to the top of every affected test file.

[DanielRosenwasser]

The following test cases would be useful too:

class Base {
    protected constructor() {
    }
}

class Derived extends Base {
    private constructor() {
        super();
    }
}

class Base {
    protected constructor() {
    }
}

class Derived extends Base {
    protected constructor() {
        super();
    }
}

var baseCtor = Base;
baseCtor = Derived;

[DanielRosenwasser]

@AbubakerB sorry this fell off of our radar, we've been hard at work on TypeScript 1.6.

@ahejlsberg @mhegazy @vladima any thoughts?

[AbubakerB]

@DanielRosenwasser I see, that's fine :)

1. I've added // @declaration: true to the three test files.
2. Added test cases under classConstructorAccessibility2.ts. Although, I have one question... Can a X ctor signature be assigned to a non-X ctor signature? (where X is public/protected/private).
3. Renamed the diagnostic message for error code 2656 to be more clear.

[DanielRosenwasser]

Shouldn't this be new (x: number): D?

[AbubakerB]

What would be the best way of adding the new before the signature output?
I can't seem to find any method that writes 'new' + signature.

[RyanCavanaugh]

@AbubakerB can you get this up-to-date and resubmit in a new PR? Very sorry we left this hanging for so long.

[AbubakerB]

Sure. I've opened a new PR for this.