Regression in optional args typecheck

[cknitt]

In ReScript 13.0.0-alpha.3, this does not compile correctly anymore:

module type Test = {
  @react.component
  let make: (~test: int=?) => React.element
}

module A: Test = {
  @react.component
  let make = (~test: option<int>=42) => {
    React.null
  }
}

It gives the error message:

This has type: int
  But it's expected to have type: option<int>

In earlier ReScript versions, it compiled fine.

(The real-world code where I extracted this simplified example from had a .res and a .resi, but for easier testing in the playground I used module type / module to be able to test with a single file.)

[cknitt]

It gets weirder, there is actually a preexisting bug that can be triggered in earlier versions (and now surfaced in the above case after the change to the JSX, too):

let f = (~test: option<int>=42) => {
  let _: int = test
}

fails with

This has type: int
  But this switch is expected to return: option<int>

  All branches in a switch must return the same type.
  To fix this, change your branch to return the expected type.

[cknitt]

Ok fixing the pre-existing bug fixes the regression, too.

[cknitt]

Ok so actually this should not be considered a regression / pre-existing issue.

For "plain" functions (without @react.component, the correct forms are:

• ~test: int=42
• ~test: option<int>=?

and everything else fails to compile.

The same should hold for functions with @react.component when they are transformed by the JSX PPX.
So the code should actually never have compiled.

The check got stricter as a side effect #8296.