Fix issue where build error of newtype escaping its scope

[mununki]

This PR fixes #5978
JSX4 generates the type declaration of props outside of make function. Any newtype expression of make function could cause the build error of escaping its scope. To fix this issue, this PR generates additional type declarations for newtypes of make function.

[cristianoc]

This declares a new type a in the current scope. Will error if a type called a is already in scope. Eg defined just above.
The (type a) should be confined to the body of make.

[mununki]

Yes, I think so too. I want to avoid to add type declaration. But I don't have a clear idea to fix this:

module type T = {
  type t
}

module V4A2 = {
  type props<'foo> = {foo: 'foo}

  let make = (type a, {foo, _}: props<'foo>) => {
    module T = unpack(foo: T with type t = a)
    foo
  }
  let make = {
    let \"Newtype$V4A2" = (props: props<_>) => make(props)

    \"Newtype$V4A2"
  }
}

[cristianoc]

If it compiles OK, should be good.
Might be worth testing a couple of examples by hand and see that it compiles OK, before changing the ppx.

[cristianoc]

That : props<'foo> should be : props<_> I think.

[mununki]

That : props<'foo> should be : props<_> I think.

Yes, It works. But How can JSX4 know that foo is going be unpack in the body expression?

// original code
module V4A2 = {
  @react.component
  let make = (type a, ~foo) => {
    module T = unpack(foo: T with type t = a)
    foo
  }
}

[mununki]

The current ppx passes the core_type of argument to props type. Do you mean passing no parameter to props type?

@react.component
let make = (~a: int) => React.int(a)

// (current) transformed to
type props<'a> = {a: 'a}
let make = ({a, _}: props<int>) => React.int(a)
                         ^^^^^

// (new?)
type props<'a> = {a: 'a}
let make = ({a, _}: props<_>) => React.int(a)
                         ^^^^^

[cristianoc]

Ah no that should stay the same.
I think above you want let make = (type a, ... : props<a>

[cristianoc]

I guess this should be let make = (type a, ...

[cristianoc]

This is now replaced by #6029 which adopts some of the suggestions from the discussion.