7.1: array-destructured parameter with a = [] default loses its contextual type (TS7031) #65069

🔎 Search Terms

TS7031, implicit any, array destructuring parameter, default value, contextual typing, padTupleType, 7.1 regression

🕗 Version & Regression Information

• This changed between versions 7.0.2 and 7.1
• Still present in typescript@7.1.0-dev.20261030.1 and current main (6161772c1)
• Related to 7.1: destructured parameter with a = {} default loses its contextual type (TS7031) #64974 (object pattern with = {}), but that issue and open fix(64974): fix contextual typing for destructured parameter defaults #64978 only cover object binding patterns. Array/= [] padding is still unconditional on main.

⏯ Playground Link

No response

💻 Code

// tsconfig: { "compilerOptions": { "strict": true, "noEmit": true, "types": [] } }
type F = (input?: [string?, number?]) => number;
export const f: F = ([a, b] = []) => (a ? 1 : 0) + (b ?? 0);

The object form from #64974 fails the same way; this report is specifically the array form:

type G = (input?: { a?: string; b?: number }) => number;
export const g: G = ({ a, b } = {}) => (a ? 1 : 0) + (b ?? 0); // also TS7031; tracked in #64974

Without the = [] initializer, the array pattern still checks cleanly on 7.1:

type F2 = (input: [string?, number?]) => number;
export const f2: F2 = ([a, b]) => (a ? 1 : 0) + (b ?? 0); // OK

🙁 Actual behavior

repro.ts(2,23): error TS7031: Binding element 'a' implicitly has an 'any' type.
repro.ts(2,26): error TS7031: Binding element 'b' implicitly has an 'any' type.

🙂 Expected behavior

No errors: a and b take string | undefined and number | undefined from the contextual parameter type, as on 7.0.2.

Additional information about the issue

In checkDeclarationInitializer, parameter array binding patterns always call padTupleType when the initializer type is a tuple. Padding invents any-typed missing elements and overrides the contextual parameter type:

case ast.KindArrayBindingPattern:
    if isTupleType(t) {
        return c.padTupleType(t, name)
    }

#64978 adds a contextual-type guard only for KindObjectBindingPattern / padObjectLiteralType. After that lands, this array case will still fail unless padTupleType gets the same getContextualTypeForVariableLikeDeclaration(root, ContextFlagsNone) == nil guard.