TypeScript build hangs when importing Fetcher type from react-router

[alexplkvf]

Reproduction

Can be reproduced in following repo:
https://stackblitz.com/edit/react-router-tsc-hangs

The project is a simple library with one function:

import type { Fetcher } from 'react-router'

export const bar = (fetcher: Fetcher) => fetcher.state === 'loading'

1. Open project
2. Run the following command npm i && npm run build

tsc process will continue to work indefinitely while actively consuming memory

So far I've reproduced it in Stackblitz environment, in MacOS environment and in node:20-alpine container

System Info

System:
    OS: Linux 6.8 Alpine Linux
    CPU: (5) arm64 unknown
    Memory: 7.19 GB / 7.73 GB
    Container: Yes
    Shell: Unknown - /bin/sh
  Binaries:
    Node: 20.20.0 - /usr/local/bin/node
    Yarn: 1.22.22 - /usr/local/bin/yarn
    npm: 10.8.2 - /usr/local/bin/npm
  npmPackages:
    react-router: 7.13.0 => 7.13.0 
    typescript: 5.9.3 => 5.9.3

Used Package Manager

npm

Expected Behavior

Simple library successfully built

Actual Behavior

Build process never finishes

[alexplkvf]

Apparently, it has something to do with recursive JsonValue type here
https://github.com/remix-run/react-router/blob/main/packages/react-router/lib/router/utils.ts#L83

Because isolating Fetcher type with all of its dependencies reproduces the issue, and removing json field from FetcherStates then fixes it.

To further narrow it down, changing the union type JsonArray = JsonValue[] | readonly JsonValue[] to type JsonArray = readonly JsonValue[] allows the build to pass. I'm not sure, but this starts to look like an issue with tsc itself. I will try to experiment with older versions.

[alexplkvf]

In the end, it boils down to this:

While emitting public declarations for bar function, tsc treats the function as type predicate for one of the states of the Fetcher. It tries then to append is operator with inlined variant of Fetcher. Because JsonValue is internal, not exported type, tsc tries to unwrap it recursively. Before version 5.7.3 there was some limit to recursion, after which it would just fall down to any. Starting with 5.7.3 there appears to be no limit, so tsc keeps trying until OOM.

Explicitly specifying return type for the function removes implicit type predicate and solves the problem, so I guess the issue should be closed. Probably, exporting JsonValue might help, should someone hit the same issue again.

Sorry for the noise.