8.0 regression: unused list variables that include side effects generate invalid JS

[TheSpyder]

Here's a simple function with an unused variable that creates a list (playground link):

let f xs =
  let unused =
    match xs with
    | Some (l) -> [l; l]
    | None  -> [1; 2]
  in
  Js.log2 "nothing to see here" xs

In the generated JS, the variable is removed completely:

function f(xs) {
  console.log("nothing to see here", xs);
}

However, if this unused variable has a side effect, the list is created but never assigned to a variable. JS arrays are allowed on lines by themselves (a weird edge case of the syntax, I guess) but objects are not.

Function with a side effect (playground link):

let f xs =
  let unused =
    match xs with
    | Some (l) -> Js.log "side effect"; [l; l]
    | None  -> [1; 2]
  in
  Js.log2 "nothing to see here" xs

v7 generates:

function f(xs) {
  if (xs !== undefined) {
    console.log("side effect");
    /* :: */[
      xs,
      /* :: */[
        xs,
        /* [] */0
      ]
    ];
  } else {
    /* :: */[
      1,
      /* :: */[
        2,
        /* [] */0
      ]
    ];
  }
  console.log("nothing to see here", xs);
}

Which is weird, but valid and works.

v8 generates:

function f(xs) {
  if (xs !== undefined) {
    console.log("side effect");
    {
      hd: xs,
      tl: {
        hd: xs,
        tl: /* [] */0
      }
    };
  } else {
    {
      hd: 1,
      tl: {
        hd: 2,
        tl: /* [] */0
      }
    };
  }
  console.log("nothing to see here", xs); 
}

Which triggers SyntaxError: Unexpected token on the tl: { line.

[bobzhang]

the fix is released in 8.0.3