Variadic Tuples that end in a literal type require explicit as const narrowing

[TrueWill]

Bug Report

🔎 Search Terms

variadic, tuple, literal, spread

🕗 Version & Regression Information

First saw in v4.9.5; have tested back to 4.4.4 and still present.

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about variadic.

⏯ Playground Link

https://www.typescriptlang.org/play?ts=4.9.5#code/MYewdgzgLgBCDWAuGBtAdB6AnAlmA5igLoA0MYArgLYBGAplkTALyoDkAhm2WzdzAEYiAbgBQogPQSYAASgQAtHQAeABzrAoSrFhBZRoSLApgV6zXQAmydJii4CxHgEI2TVik782rkeMPQMADuevAcuibWqBho2HiEpDA+bizsXC5sMBwQMAFQfgbggSFY8BA2cQQZ7mnevmJAA

💻 Code

const ok: [...string[], number] = ['a', 'b', 1];

// @ts-expect-error
const unexpected: [...string[], '!'] = ['a', '!'];

const workaround: [...string[], '!'] = ['a', '!' as const];

const works: [string, '!'] = ['a', '!'];

🙁 Actual behavior

For Variadic Tuples that end in a literal type, assigning a matching Tuple gives

Type '[string, string]' is not assignable to type '[...string[], "!"]'.
Type at position 1 in source is not compatible with type at position 1 in target.
Type 'string' is not assignable to type '"!"'.

🙂 Expected behavior

For normal Tuples and Variadic Tuples that only begin with a literal type, assignments do not need as const to prevent widening.

For example this works:

const works2: ['!', ...string[], ] = ['!', 'x'];

It would be more intuitive if all Tuples behaved in the same way.

[swachter]

I found a similar issue:

interface Test {
    s: 'a'
}

const works: Test[] = [{ s: 'a'}]

// TS2322: Type '{ s: string; }' is not assignable to type 'Test'.
//   Types of property 's' are incompatible.
//     Type 'string' is not assignable to type '"a"'.
const fails: Test[] = [...[{ s: 'a'}]]

[TrueWill]

This does appear to be a duplicate of #52684 as per @Andarist.

[ahejlsberg]

This actually isn't a duplicate of #52684, though the issues are closely related. I will modify #52769 to also handle this one.