[2.0] createContext(undefined) throws ContextNotFoundError, even when a provider passes undefined #4181

Describe the bug

getContext (packages/signals/src/core/context.ts) treats undefined as "not found":

let value = owner._context[context.id];
if (value === undefined) value = context.defaultValue;
if (value === undefined) throw new ContextNotFoundError();

So createContext<T | undefined>(undefined), the most common optional-context idiom in React and Solid 1 (where useContext returned undefined), throws when read outside a provider. It also throws when a provider explicitly passes undefined. Inside a computation, the throw halts reactivity for the whole app (#4178).

The message, "Context must either be created with a default value or a value must be provided before accessing it", sounds wrong when the caller did pass a default.

Your Example Website or App

Minimal repro inline below (signals-only).

Steps to Reproduce the Bug or Issue

import { createContext, getContext, setContext, createRoot, createSignal, createRenderEffect, flush } from "@solidjs/signals";

const Opt = createContext<string | undefined>(undefined);
const OptNull = createContext<string | null>(null);

createRoot(() => getContext(Opt));                              // throws ContextNotFoundError
createRoot(() => { setContext(Opt, "x"); getContext(Opt); });   // "x"
createRoot(() => { setContext(Opt, undefined); getContext(Opt); }); // throws ContextNotFoundError
createRoot(() => getContext(OptNull));                          // null

// Inside a computation:
const [show, setShow] = createSignal(false);
const [b, setB] = createSignal(0);
createRoot(() => createRenderEffect(b, v => console.log("b", v)));
createRoot(() => createRenderEffect(() => (show() ? getContext(Opt) : null), () => {}));
flush();
setShow(true); flush(); // StatusError (ContextNotFoundError) + [REACTIVITY_HALTED]
setB(1); flush();       // "b 1" never logs

Expected behavior

• Distinguish "no default argument" from an explicit undefined default (arguments.length or a sentinel), and throw only for the former.
• Treat a provider's explicit value={undefined} as a provided value.
• If that's not wanted, at minimum say in the error message and the docs/cheatsheet that undefined counts as missing, and that null is the way to make a context optional.

Screenshots or Videos

No response

Platform

• OS: macOS (Darwin 25.5)
• Browser: n/a (reproduced in Node v24.18.0 with vitest, importing source)
• Version: solid next @ b0ad1df (2026-10-20, after 2.0.0-rc.16)

Additional context

This came up while verifying examples for the From React docs additions (solidjs/solid-docs#1664). It's also a silent migration break from 1.x, where useContext returned undefined.

— Claude, on behalf of @devagrawal09