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Ramanpreet Nara 572e1da889 Fix type alias nullability resolution in module parser

Summary:
Consider this case:

```
type Animal = ?{|
  name: string,
|};

type B = Animal

export interface Spec extends TurboModule {
  +greet: (animal: B) => void;
}
```

The generated output for this spec is:

```
namespace JS {
  namespace NativeSampleTurboModule {
    struct Animal {
      NSString *name() const;

      Animal(NSDictionary *const v) : _v(v) {}
    private:
      NSDictionary *_v;
    };
  }
}

protocol NativeSampleTurboModuleSpec <RCTBridgeModule, RCTTurboModule>
- (void)greet:(JS::NativeSampleTurboModule::Animal &)animal;
end
```

Observations:
  1. The codegen looks as though we wrote `+greet: (animal: ?Animal) => void;` as opposed to `+greet: (animal: B) => void;`
  2. The generated struct is called `Animal`, not `B`.

## After this diff
Whenever we detect a usage of a type alias, we recursively resolve it, keeping a track of whether the resolution will be nullable. In this example, we follow B to Animal, and then Animal to ?{|name: string|}.

Then, we:
  1. Replace the `B` in `+greet: (animal: B) => void;` with `?Animal`,
  2. Pretend that `Animal = {|name: string|}`.

Why do we make all type alias RHSs required?
 2. This design is simpler than managing nullability in both the type alias usage, and the type alias RHS.
 3. What does it mean for a C++ struct, which is what this type alias RHS will generate, to be nullable? ¯\_(ツ)_/¯. Nullability is a concept that only makes sense when talking about instances (i.e: usages) of the C++ structs. Hence, it's better to manage nullability within the actual TypeAliasTypeAnnotation nodes, and not the associated ObjectTypeAnnotations.

## Other Changes
- Whenever we use the `Animal` type-alias, the e2e jest tests validate that the type alias exists in the module schema.

Changelog: [Internal]

Reviewed By: PeteTheHeat

Differential Revision: D23225934

fbshipit-source-id: 8316dea2ec6e2d50cad90e178963c6264044f7b7

2020-09-29 14:39:39 -07:00