Factory generated constructors are not detected as pure

[KimlikDAO-bot]

In most cases, GCC is able to figure out if a constructor is pure. As an example, consider the Vec class, which is a 2 dimensional vector.

/** @interface */
class IVec {
  /** @const {number} */
  x;
  /** @const {number} */
  y;
}

/** @implements {IVec} */
class Vec {
  /**
   * @param {number} x
   * @param {number} y
   */
  constructor(x, y) {
    /** @const {number} */
    this.x = x;
    /** @const {number} */
    this.y = y;
  }
}

new Vec(2, 2);

When compiled, this will produce no code, since the constructor is correctly determined to be pure.

When the class is generated through a factory, this inference does not seem to work. Consider for instance a factory which generates a class of points on a certain ray. Using this factory, we create a class DiagonalVec, which is the set of points with x=y.

/** @interface */
class IVec {
  /** @const {number} */
  x;
  /** @const {number} */
  y;
}

/**
 * @param {number} slope
 * @return {function(new:IVec, number)}
 */
const fixedSlopeFactory = (slope) => /** @implements {IVec} */ class {
  /** @param {number} x */
  constructor(x) {
    /** @const {number} */
    this.x = x;
    /** @const {number} */
    this.y = slope * x;
  }
}

/** @const {function(new:IVec, number)} */
const DiagonalVec = fixedSlopeFactory(1);

new DiagonalVec(2);

Now if we compile this using

 bun google-closure-compiler -O ADVANCED --js a.js --jscomp_error strictCheckTypes --jscomp_error reportUnknownTypes --language_in UNSTABLE

the compiler outputs new ((b=>class{constructor(a){this.x=a;this.y=b*a}})(1))(2); In particular, the factory generated constructor was not proven to be pure.

How can I make the compiler figure this out, potentially using unsafe annotations such as @pureOrBreakMyCode ?