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Type inference failure when spreading a readonly array type parameter #33282

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@whatisaphone

Description

@whatisaphone

TypeScript Version: 3.5.1 and 3.7.0-dev.20190906

Search Terms: spread readonly array sadness

Code

export class Event<A extends ReadonlyArray<unknown>> {
    public attach(listener: (...args: A) => void): void { }
}

export function doAttach<A extends ReadonlyArray<unknown>>(
    event: Event<A>,
    listener: (...args: A) => void,
): void { }

function theRealTest(
    event: Event<readonly [number]>,
    listener: (_: number) => void,
) {
    doAttach(event, listener);
}

Expected behavior:

Successful compilation.

Actual behavior:

test.ts:14:14 - error TS2345: Argument of type 'Event<readonly [number]>' is not assignable to parameter of type 'Event<[number]>'.
  The type 'readonly [number]' is 'readonly' and cannot be assigned to the mutable type '[number]'.

14     doAttach(event, listener);
                ~~~~~

Playground Link: You got it!

Armchair analysis:

The error message mentions Event<[number]>, but that type doesn't actually exist in the source code. The type should be Event<readonly [number]>. Somewhere along the line, the type is losing its readonly-ness.

The code compiles fine if you write out the correct type parameter:

    doAttach<readonly [number]>(event, listener);

Shot in the dark: Maybe the A in doAttach is being inferred as a non-readonly array due to the spread in the type of listener?

Related Issues:

I could not find any related issues.

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