4.7 regression: Array.flat(infinity) leads to "Type instantiation is excessively deep and possibly infinite." error

Bug Report

🔎 Search Terms

• Array.flat
• Type instantiation is excessively deep and possibly infinite.
• Nested arrays

🕗 Version & Regression Information

• This changed between versions 4.6 and 4.7

⏯ Playground Link

TypeScript Workbench with relevant code

💻 Code

interface Config {
  prop?: string;
}

interface ExtendedConfig extends Config {}

type NestedConfigs = Array<ExtendedConfig | NestedConfigs>;

const configs: NestedConfigs[] = [];

const flattened = configs.flat(Infinity);

🙁 Actual behavior

TypeScript raises the following error for the configs.flat(Infinity) call:

Type instantiation is excessively deep and possibly infinite. ts(2589)

🙂 Expected behavior

No error should be raised.

[taylorfsteele]

Just ran into this as well, here’s a very concise example:

type StringOrStringArray = string | StringOrStringArray[];

const flatten = (...args: StringOrStringArray[]) => args.flat(Infinity);

Same exact error on args.flat(Infinity), “Type instantiation is excessively deep and possibly infinite.”

A quick solution for now is to assert your .flat argument as an integer:

args.flat(Infinity as 10)

Should make the error go away.

[tjjfvi]

This could be addressed if a literal type for Infinity was added: playground link using 1e309