Object destructuring with a default over a source lacking the property reports spurious 'Property does not exist' (declaration + assignment)

[nickna]

Summary

Destructuring an object property that carries a default, over a source whose static type does not declare that property, reports a spurious Property 'x' does not exist on type '...' — even though the default makes the access safe. tsc accepts these (the binding pattern contributes an optional property as contextual type to the source).

const { a = 5 } = {};                       // Type Error: Property 'a' does not exist on type '{ }'.
let b; ({ b = 5 } = {});                     // same (assignment form, #780)
let x; ({ a: x = 5 } = {});                  // same (existing #754 rename form)
({ p: { x } = {} } = { p: { x: 1 } });       // same: inner default `{}` lacks `x` (#779)

A source that declares the property (even optional) works:

const o: { a?: number } = {};
const { a = 5 } = o;                          // OK -> a === 5

Cause

SharpTS does not thread the binding/assignment pattern as a contextual type to the destructuring source. The desugared property read (_src.a, via Expr.Get) is type-checked against the source's declared type; a closed object type ({}, or any type missing the key) has no a, so CheckGet reports the missing-property error. tsc instead types the source under the pattern's implied type, in which a defaulted property is optional, so _src.a reads as T | undefined and the default covers it.

Note the asymmetry with arrays: the desugared array index _src[i] is OOB-tolerant in the checker (no error), so const [a = 5] = [] already works; only the object property path errors. A property without a default correctly stays an error (const { a } = {} is a tsc error too) — the leniency must apply only when a default is present.

Scope

Pre-existing; affects declaration destructuring and the shipped #754 assignment-destructuring forms. Surfaced again while completing #780 (object shorthand-with-default parsing) and #779 (nested pattern with default) — the parser/lowering for those is correct, but their canonical empty-source/empty-default examples hit this type error. Same root cause family as #783 (no binding-pattern contextual typing, there for array-literal tuple inference).

Fix shape

Synthesize the pattern's implied source type (defaulted/optional properties) and type the source under it as a contextual type — or, more narrowly, make a defaulted destructuring property read tolerate a missing property (treat it as propertyType | undefined) while leaving a non-defaulted read strict (matching tsc).