RangeError: Maximum call stack size exceeded when trying to infer type of a variable reassigned in a loop

[afarnsworth-valve]

🔎 Search Terms

"Maximum call stack size exceeded"
"Maximum call stack size exceeded loop"
"loop reassign referenced in its own initializer"

🕗 Version & Regression Information

• This changed between versions 5.3.3 and 5.4.5

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.9.3#code/JYOwLgpgTgZghgYwgAgHIHsAmECwAoAb3wEgEALYAG0yghAH4AuZWuTdESgTzSwgG0AugG4SobAA8myEAFcAtgCNoovAF98+GLJAIwwDsgCSkKHEgZsAZwAUyTObjMCMvs0so1yAJT4ieYkoIMFdsdz5kAB9kHWwYUAhMZABee0cAOhA+VWIAdwog5Dss7B8Sf2JSDisQl3IqGjoAGmRxCAkU5ABaAEZkL1SSiByqkBrkRH0ANwgPcNLo2Ih4kETOto6AQlTe5AAyPeR66loQZHojgsaQfg3BZGYllcTVEmIhzsngGY8cjXUgA

💻 Code

interface Node
{
	children?: readonly Node[];
	index?: number;
}

function IterateNodes( data: { node: Node } )
{
	let node: Node | undefined = data.node;
	while ( node )
	{
		const { children, index = -1 } = node;
		const activeNode: Node | undefined = index != -1 && children ? children[index] : undefined;

		node = activeNode;
	}
}

🙁 Actual behavior

tsc will crash with a stack like:

RangeError: Maximum call stack size exceeded
    at checkExpressionWorker (D:\src\steam\main\web_src\node_modules\typescript\lib\typescript.js:85502:33)
    at checkExpression (D:\src\steam\main\web_src\node_modules\typescript\lib\typescript.js:85457:32)
    at checkTruthinessExpression (D:\src\steam\main\web_src\node_modules\typescript\lib\typescript.js:88209:34)
    at checkConditionalExpression (D:\src\steam\main\web_src\node_modules\typescript\lib\typescript.js:85005:18)
    at checkExpressionWorker (D:\src\steam\main\web_src\node_modules\typescript\lib\typescript.js:85595:16)
    at checkExpression (D:\src\steam\main\web_src\node_modules\typescript\lib\typescript.js:85457:32)
    at getTypeOfExpression (D:\src\steam\main\web_src\node_modules\typescript\lib\typescript.js:85398:18)
    at getTypeOfInitializer (D:\src\steam\main\web_src\node_modules\typescript\lib\typescript.js:74377:34)
    at getInitialTypeOfVariableDeclaration (D:\src\steam\main\web_src\node_modules\typescript\lib\typescript.js:74381:14)
    at getInitialType (D:\src\steam\main\web_src\node_modules\typescript\lib\typescript.js:74392:58)

The actual stack varies a bit but generally these functions.

Prior to typescript 5.4.5, this would not crash but would error that children and index are referenced in their own initializer. All of these errors occur only in strict mode.

🙂 Expected behavior

tsc should not crash but display errors (even though these are somewhat confusing errors)

Additional information about the issue

Was able to resolve the errors by changing the loop variable from let node: NodeType | undefined = data.node; to let node = data.node as NodeType | undefined (playground)

[RyanCavanaugh]

Slightly simplified version; repros in TS7

interface MyNode {
  children: MyNode[];
  index?: number;
}

function f(init: MyNode) {
  let node: MyNode | undefined = init;
  while (node) {
    const { children, index = 0 } = node;
    node = children[index];
  }
}