async function with @this attribute

[Mepy]

// ReScript
let add = x=>async(y)=> x+y
let method = @this this=> async arg=>await this->add(arg)

In rescript v10.1.3, the example above will fail as the following :

FAILED: 
  We've found a bug for you!

  1 │ // ReScript
  2 │ let add = x=>async(y)=> x+y
  3 │ let method = @this this=> async arg=>await this->add(arg) 
  4 │ 

  Await on expression not in an async context

By the way, the following ReScript code will generate JavaScript code without async:

// ReScript
let method = @this this=> async arg=>this+arg

// JavaScript
function method(arg) {
  var $$this = this ;
  return $$this + arg | 0;
}

[cristianoc]

Thank you for reporting this.
Likely the compiler's ppx implementation of @this eats up the async attribute. And it always did. Only we did not use attributes before (an attribute is how async looks like in the AST).

[cristianoc]

To verify the hypothesis, let's put the second example in tst.res and do:

./bsc -dsource tst.res   
let method =
  Js_OO.unsafe_to_method
    ({ I2 = (fun this -> fun arg -> this + arg) } : _ Js_OO.Callback.arity2)

there's no attribute.

Instead a single async function would look like this internally:

let foo = ((fun arg -> Js.Promise.unsafe_async (arg + 1))[@res.async ])

So this seems to confirm the hypothesis.

[cristianoc]

Looking at this cheat sheet, the @this decorator was left out. As it's the most complex of them all.
In a future clean up that decorator would be the first to go.
But for now, let's look into fixing it.

[cristianoc]

Looks like this https://github.com/rescript-lang/rescript-compiler/blob/master/jscomp/frontend/ast_core_type_class_type.ml#L84 is the pace where it is handled.

[cristianoc]

Here's the fix:
#6105

The issue and fix are as expected.