Default function parameters typed as undefined when enclosed in an internal function

[hills]

I think this looks like a bug in the compiler, see the minimal test case below.

The function parameter has a default, so it may never be "undefined" and the compiler honours this.

What I didn't expect is that enclosed in a function it somehow it picks up the possibility to be "undefined".

It happens for a range of types; function, string etc. The case where I noticed was a callback function provided to a constructor.

If I'm mistaken and this is not a bug but actually some edge case of the language then I'd also be open to hear of that. Many thanks.

/**
 * @param {string} v
 */

function outer(v = 'fallback') {
    inner(v);   //  <-- string

    function local() {
        inner(v);    //   <-- string|undefined ???
    }
}

/**
 * @param {string} v
 */

function inner(v) {
    console.log(v);
}

outer();

$ java -jar closure-compiler-v20230502.jar -O ADVANCED test.js
test.js:9:14: WARNING - [JSC_TYPE_MISMATCH] actual parameter 1 of inner does not match formal parameter
found   : (string|undefined)
required: string
   9|         inner(v);
                    ^

0 error(s), 1 warning(s), 100.0% typed
(function(a="fallback"){console.log(a)})();