Allow inheritance from intersections

[sandersn]

Based on #14017, when exporting a class that extends from a mixin, the emitted type contains an intersection. Inheriting from an intersection is not allowed, even though it should be.

Note that inheriting from a type alias of an intersection already works:

// A *self-contained* demonstration of the problem follows...
class A { a = 1 } 
class B { b = 2 }
type Both = A & B
class C extends A & B { } // error
class D extends Both { } // ok

Expected behavior:
No error for C or D.

Actual behavior:
Error for C.

[ahejlsberg]

You're confusing types and expressions in your example. The extends clause of a class requires an expression, not a type, so C and D are both errors. The right way to write the example is

class A { a = 1 } 
class B { b = 2 }
declare const Both: new () => A & B;
class D extends Both { }

This of course omits the details of how A and B are actually intersected.

The declaration file emitter currently depends on the fact that a class declares both a type and a value. It actually emits the extends clause as a type, even though a value is required in that position. We've gotten away with it so far because classes typically inherit from entities that share the same name for a type and a value, but that's no longer the case with mixins.

[mhegazy]

Allowing a type in an ambient declaration should be fine though.

[ahejlsberg]

Not sure how that would work. First, it would be ambiguous how to parse it. Second, we require an expression of a constructor type, not an instance type, so you couldn't just write A & B as those are instance types.

[mhegazy]

Either we allow inheriting from types in ambient declarations, or we need to write a declare const before a class with a type that has no name.