NTI @template THIS issue

[supersteves]

Trying to use covariant return type like this, in NTI:

  /** @param {Object} obj */
  var helper = function(obj) {}; // in externs

  /**
   * @this {THIS}
   * @return {THIS}
   * @template THIS
   */
  x() {
    this.something();
    helper(this);
    return this;
  }

Warnings generated:

input0:9: WARNING - Cannot access property something of non-object type THIS.
    this.something();
    ^

input0:10: WARNING - Invalid type for parameter 1 of function helper.
expected : Object|null
found    : THIS

    helper(this);

I think both should be inferred as valid.

https://closure-compiler-debugger.appspot.com/#input0%3Dclass%2520X%2520%257B%250A%250A%2520%2520%252F**%250A%2520%2520%2520*%2520%2540this%2520%257BTHIS%257D%250A%2520%2520%2520*%2520%2540return%2520%257BTHIS%257D%250A%2520%2520%2520*%2520%2540template%2520THIS%250A%2520%2520%2520*%252F%250A%2520%2520x()%2520%257B%250A%2520%2520%2520%2520this.something()%253B%250A%2520%2520%2520%2520helper(this)%253B%250A%2520%2520%2520%2520return%2520this%253B%250A%2520%2520%257D%250A%250A%2520%2520something()%2520%257B%2520%250A%2520%2520%2520%2520console.log(this)%253B%2520%250A%2520%2520%257D%250A%250A%257D%26input1%26conformanceConfig%26externs%3D%252F**%2520%2540param%2520%257BObject%257D%2520obj%2520*%252F%250Avar%2520helper%2520%253D%2520function(obj)%2520%257B%257D%253B%250A%26refasterjs-template%26includeDefaultExterns%3D1%26CHECK_SYMBOLS%3D1%26CHECK_TYPES%3D1%26CHECK_TYPES_NEW_INFERENCE%3D1%26LANG_IN_IS_ES6%3D1%26MISSING_PROPERTIES%3D1%26PRESERVE_TYPE_ANNOTATIONS%3D1%26PRETTY_PRINT%3D1%26TRANSPILE%3D1

Related to #1411

[dimvar]

The right fix for this is to handle bounded generics in NTI. Without that, we probably want to warn here. We don't know anything about the type variable, we don't know if it's an object, or what properties it has. So, without bounded generics, you have to cast for the time being.

[supersteves]

#576

[supersteves]

Thanks @dimvar