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Scanner's sources and build's candidates refuse a readonly array, though each is only read #20706
Description
What version of Tailwind CSS are you using?
v4.3.5, and the insiders build of main (0.0.0-insiders.a3cb19c)
What build tool (or framework if it abstracts the build tool) are you using?
None: TypeScript 7.0.2 (tsc) type-checking code that calls @tailwindcss/node and @tailwindcss/oxide
What version of Node.js are you using?
v26.12.1
What browser are you using?
N/A
What operating system are you using?
Windows 11
Reproduction URL
https://github.com/YevheniiKotyrlo/tailwindcss-readonly-inputs
Describe your issue
@tailwindcss/oxide declares interface ScannerOptions { sources?: Array<SourceEntry> }, and the objects compile from tailwindcss and from @tailwindcss/node resolve to declare build(candidates: string[]). The scanner only reads its sources and build only iterates its candidates, but code that holds either as a readonly array can hand it to neither without copying it into a mutable one:
export const scanned = (sources: readonly SourceEntry[]): readonly string[] => new Scanner({ sources }).scan();
export const builtWithNode = async (css: string, candidates: readonly string[]): Promise<string> => {
const compiler = await compile(css, { base: process.cwd(), onDependency: () => undefined });
return compiler.build(candidates);
};git clone https://github.com/YevheniiKotyrlo/tailwindcss-readonly-inputs && cd tailwindcss-readonly-inputsnpm installnpm run check, which type-checksindex.tswithstrict: true.
Expected: no errors: ScannerOptions.sources takes a readonly SourceEntry[] and build a readonly string[], while a mutable array still type-checks at both.
Actual, on the insiders build of main:
System: MINGW64_NT-10.0-26300, node v26.12.1, npm 12.2.1, tailwindcss 0.0.0-insiders.a3cb19c, @tailwindcss/node 0.0.0-insiders.a3cb19c, @tailwindcss/oxide 0.0.0-insiders.a3cb19c, typescript 7.0.2
index.ts(5,94): error TS4104: The type 'readonly SourceEntry[]' is 'readonly' and cannot be assigned to the mutable type 'SourceEntry[]'.
index.ts(9,25): error TS4104: The type 'readonly string[]' is 'readonly' and cannot be assigned to the mutable type 'string[]'.
index.ts(14,25): error TS4104: The type 'readonly string[]' is 'readonly' and cannot be assigned to the mutable type 'string[]'.
SUMMARY: defect: 3 call(s) refuse a readonly array, taking only a mutable one
npm run check exits 1 while any of the three refuses the readonly array and 0 once index.ts type-checks. The declarations come from crates/node/src/lib.rs (pub sources: Option<Vec<SourceEntry>>, which NAPI-RS writes as Array<SourceEntry>) and packages/tailwindcss/src/index.ts (build(candidates: string[])).
Activity
YevheniiKotyrlo commentedon Nov 11, 2026
The patches I apply to the published 4.3.3 declarations, which type the three as taking a readonly array: