Variables declared in array destructuring patterns can't be used in initializers of other variables in the same pattern

[thorn0]

TypeScript Version: 3.6.2

Search Terms: circular reference, array destructuring initializer

Code

// @strict
declare const x: number | undefined;
const [a, b = a] = [10, x];

Expected behavior: Should compile. Both a and b are of type number.

Actual behavior:
Error: 'a' implicitly has type 'any' because it does not have a type annotation and is referenced directly or indirectly in its own initializer.

Playground Link: playground

Related Issues: #32950, #33191

[sandersn]

Pretty sure this is a design limitation of the way that Typescript checks types. It uses a simple strategy augmented by contextual typing, but that is simple too, and happens at the same time during checking every time.

Still, it might be possible to tweak the order of checking in destructuring declarations to avoid the error.

[typescript-bot]

👋 Hi, I'm the Repro bot. I can help narrow down and track compiler bugs across releases! This comment reflects the current state of the repro in the issue body running against the nightly TypeScript.

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Issue body code block by @thorn0

👍 Compiled

Historical Information

Version	Reproduction Outputs
5.4.2	👍 Compiled
5.0.2, 5.1.3, 5.2.2, 5.3.2	❌ Failed: - 'a' implicitly has type 'any' because it does not have a type annotation and is referenced directly or indirectly in its own initializer.