Experiment on Closure Compiler's function types (behaviour of all '*' vs. unknown '?')

functions.js

// Run with:
// java -jar compiler.jar --language_in=ECMASCRIPT6_STRICT -O ADVANCED --warning_level VERBOSE functions.js 

/**
 * @param {function(*)} f1 ERROR
 * @param {function(*):*} f2 ERROR
 * @param {function(?):*} f3
 * @param {function(*):?} f4 ERROR
 * @param {function(?):?} f5
 * @param {function()} f6 ERROR
 * @param {Function} f7
 * @param {function(number):number} f8
 */
function higherOrder(f1, f2, f3, f4, f5, f6, f7, f8) {
  console.log(f1 + f2 + f3 + f4 + f5 + f6 + f7 + f8);
}

/** @return {number} @param {number} x */
function f(x) { return x + 1; };

higherOrder(/* ERROR */ f, /* ERROR */ f, f, /* ERROR */ f, f, /* ERROR */ f, f, f);



/**
 * @param {function({x: (*|undefined)})} f1
 * @param {function({x: (?|undefined)})} f2
 */
function higherOrderNamed(f1, f2) {
  console.log(f1 + f2);
}

/** @param {{x: (*|undefined)}} opts */
function fany(opts) { return opts.x; };
/** @param {{x: (number|undefined)}} opts */
function fnumber(opts) { return opts.x; };

higherOrderNamed(fany, fany);
higherOrderNamed(/* ERROR */ fnumber, fnumber);