Can't parse foo! += 1

[paoloricciuti]

If i try to parse the expression in the title i get an error despite it being a valid typescript syntax.

Syntax:

let foo: number | null  = 1;
foo! += 1;

Error:

Error: Assigning to rvalue (3:0)

Repo:
link

Related issue:

sveltejs/svelte#13409

We are using acorn-typescript in svelte to parse typescript scripts and this bug was reported.