Global T | null | undefined initialised from a T | undefined call: 'ts.Global' op expects region #0 to have 0 or 1 blocks #462

A module-level variable of type T | null | undefined, initialised from a call that returns T | undefined, fails to compile in every memory model.

class Node { v = 0; tag = ""; }
function mk(v: number) { const n = new Node(); n.v = v; n.tag = "t" + v; return n; }
function mkU(): Node | undefined { return mk(7); }
let g: Node | null | undefined = mkU();
function main() { print("cls u", g ? g.tag : "none"); }

tslang --emit=jit --no-default-lib -mm=gc (rc the same), main 8b9d631:

error: 'ts.Global' op expects region #0 to have 0 or 1 blocks
let g: Node | null | undefined = mkU();

Expected: cls u t7.

If mkU is declared to return Node | null | undefined instead, the program compiles, with a warning casting from optional (undefined) value, and prints 1. So the problem is converting an optional (T | undefined) to the T | null | undefined union inside a global initializer, which apparently produces control flow that a ts.Global region cannot hold. Found by the review of Shared<T> (own phase 8). Shared<T> | null | undefined fails the same way.

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