-mm=rc: const b = a (a let of a class) takes no reference; reassigning a frees what b holds #454

Under -mm=rc, const b = a, where a is a let holding a class instance, does not keep the instance alive. Reassigning a frees the object while b still refers to it.

class Node { v = 0; name = ""; }

function main() {
    let a = new Node();
    a.name = "first" + 1;
    const b = a;
    a = new Node();
    let keep: string[] = [];
    for (let i = 0; i < 1000; i++) keep.push("k" + i);
    assert(b.name == "first1", "b still holds the first node");
    print("done.");
}

tslang --emit=jit --no-default-lib, main 8b9d631:

• gc and none: done.
• rc: assertion failed: b still holds the first node, exit code 3. The first Node and its string are freed when a is reassigned, and the keep loop reuses the memory.

The same program with only a number field (v) passes under rc, because the freed block is not reused before b.v is read. The bug is still there, it just does not show.

Probable cause: a const initialised from a let is folded into a read of the let rather than given storage and a reference of its own. Branch own-shared (own phase 8) fixed this for Shared<T> handles only, by giving a const of that type identity storage (needsIdentityStorage, beside ArrayType). Class instances still need the fix.

🤖 Generated with Claude Code