Proposal: The internal modifier for classes

The internal modifier

Often there is a need to share information on types within a program or package that should not be
accessed from outside of the program or package. While the public accessibility modifier allows
types to share information, is insufficient for this case as consumers of the package have access
to the information. While the private accessibility modifier prevents consumers of the package
from accessing information from the type, it is insufficient for this case as types within the
package also cannot access the information. To satisfy this case, we propose the addition of the
internal modifier to class members.

Goals

This proposal aims to describe the static semantics of the internal modifier as it applies to members of a class (methods, accessors, and properties).

Non-goals

This proposal does not cover any other use of the internal modifier on other declarations.

Static Semantics

Visibility

Within a non-declaration file, a class member marked internal is treated as if it had public
visibility for any property access:

source.ts:

class C {
    internal x: number;
}

let c = new C();
c.x = 1; // ok

When consuming a class from a declaration file, a class member marked internal is treated as
if it had private visibility for any property access:

declaration.d.ts

class C {
    internal x: number;
}

source.ts

/// <reference path="declaration.d.ts" />
let c = new C();
c.x = 1; // error

Assignability

When checking assignability of types within a non-declaration file, a class member marked
internal is treated as if it had public visibility:

source.ts:

class C {
    internal x: number;
}

interface X {
    x: number;
}

let x: X = new C(); // ok

If one of the types is imported or referenced from a declaration file, but the other is defined
inside of a non-declaration file, a class member marked internal is treated as if it had
private visibility:

declaration.d.ts

class C {
    internal x: number;
}

source.ts

/// <reference path="declaration.d.ts" />
interface X {
}

let x: X = new C(); // error

It is important to allow assignability between super- and subclasses from a declaration file
with overridden members marked internal. When both types are imported or referenced from a
declaration file, a class member marked internal is treated as if it had protected visibility:

declaration.d.ts

class C {
    internal x(): void;
}

class D extends C {
    internal x(): void;
}

source.ts

/// <reference path="declaration.d.ts" />
let c: C = new D(); // ok

However, this does not carry over to subclasses that are defined in a non-declaration file:

declaration.d.ts

class C {
    internal x(): void;
}

source.ts

/// <reference path="declaration.d.ts" />
class D extends C {
    internal x(): void; // error
}

Start by studying the proposal's static-semantics sections and its examples for member visibility and assignability in declaration and non-declaration files. Done means the internal modifier behaves consistently with the stated public, private, and protected rules, including overridden members and the non-declaration subclass case.