Object shorthand-with-default `{a = 5}` is rejected by the object-literal parser (blocks that form of assignment destructuring)

[nickna]

Summary

The object-literal parser rejects a shorthand property with a default:

let a;
({ a = 5 } = {});      // Parse Error: Expect '}' after object literal.

This is the object-pattern cover-grammar form. Other object assignment-destructuring forms parse and work after #754: shorthand {a}, rename {a: x}, rename+default {a: x = 5}, nested {a: [x]}, rest {...r}, computed {[k]: v}, string-key {"a-b": v}.

Cause

ParseObjectLiteral does not accept = after a shorthand identifier key ({ a = 5 } is only valid as a destructuring pattern, not as an object literal expression). With no cover grammar, the LHS never parses, so the = dispatch in Parser.Expressions.cs is never reached.

Fix shape

Allow shorthand-with-default in the object-literal parser (storing it as { a: (a = 5) } shape, value = Expr.Assign) so it round-trips to the #754 assignment-destructuring lowering (which already handles a {a: x = 5} rename+default). The pure-expression use of {a = 5} remains a semantic error, as in tsc.