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

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难度
4/5
预计耗时
3-5 天
新手友好度
56/100
Issue 类型
缺陷
描述清晰度
描述清楚
活跃度
活跃
技术栈
javascript, typescript
领域
api, backend

调研方向

Start in packages/signals/src/core/context.ts and read getContext alongside the reported examples. Reproduce the cases for an explicit undefined default and provider value, then check how context errors affect computations. Done means the chosen behavior matches the expected cases and the error or docs explain the undefined behavior; the issue does not name a test file.

由索引模型根据 Issue 内容生成。

描述

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 @ b0ad1df50 (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.

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  4. 提交 Pull Request，并在描述里引用这个 Issue 编号。

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