usage of closure cannot be diagnosed in assignment statement

[HerrCai0907]

Bug description

usage of closure cannot be diagnosed in assignment statement

Steps to reproduce

compiler following code

function foo(): () => void {
  let a = 100;
  let bar = (): void => {
    a = 10;
    return;
  };
  return bar;
}

foo();

And then got validate error

[wasm-validator error in function assembly/index/foo~anonymous|0] unexpected false: local.set index must be small enough, on 
(local.set $0
 (i32.const 10)
)
FAILURE /Users/assemblyscript/cli/index.js:770
      return prepareResult(Error("validate error"));

AssemblyScript version

v0.27.x

[HerrCai0907]

It will cause some weird bug in some case

function foo(): () => void {
  let a = 100;
  let bar = (): void => {
    let b = 1;
    a = 10;
    trace("b =", 1, b);
    return;
  };
  return bar;
}

export function _start(): void {
  let bar = foo();
  bar();
}

When you call _start() then you will get b = 10 🤣