Fix unique symbol widening in expando property assignments

[Copilot]

Unique symbols assigned as expando properties (e.g., Objs.Sym = Sym) were incorrectly widened from unique symbol to symbol, breaking type narrowing via equality checks.

const Sym: unique symbol = Symbol("test");
const Objs = () => {};
Objs.Sym = Sym;

type Val = { attr: 1 } | typeof Objs.Sym;
const vals: Val[] = [Objs.Sym];
console.log(vals[0] != Objs.Sym && vals[0].attr); // TS2339 in tsgo, works in tsc

Cause

getAssignmentDeclarationInitializerType used checkExpressionForMutableLocation, which calls getWidenedLiteralLikeTypeForContextualType → getWidenedUniqueESSymbolType, widening unique symbol to symbol. TypeScript's getInitializerTypeFromAssignmentDeclaration uses getWidenedLiteralType which does not widen unique symbols.

Fix

• Replaced checkExpressionForMutableLocation in getAssignmentDeclarationInitializerType with inlined widening logic that applies getWidenedLiteralType (preserving contextual type handling for regular literals) but omits the getWidenedUniqueESSymbolType call.
• Added compiler test uniqueSymbolPropertyNarrowing.ts covering both property-access and direct-identifier narrowing cases.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Unique symbol defined as attribute not narrowed</issue_title>
<issue_description>## Steps to reproduce

Behaviour difference between 6.0 and 7.0. The following snippet compiles without errors on earlier versions (which I believe is correct) but fails in the native preview with the below error.

const Sym: unique symbol = Symbol("test");
const Objs = () => {};
Objs.Sym = Sym;

type Val = { attr: 1} | typeof Objs.Sym;
const vals: Val[] = [Objs.Sym];
console.log(vals[0] != Objs.Sym && vals[0].attr);

Note that the following similar snippet works without error in both versions:

const Sym: unique symbol = Symbol("test");

type Val = { attr: 1 } | typeof Sym;
const vals: Val[] = [Sym];
console.log(vals[0] != Sym && vals[0].attr);

Behavior with [email protected]

$ npm exec -- tsc --noEmit test.ts && echo Success
Success

Behavior with tsgo

$ npm exec -- tsgo --noEmit test.ts && echo Success
test.ts:7:44 - error TS2339: Property 'attr' does not exist on type 'Val'.
  Property 'attr' does not exist on type 'symbol'.

7 console.log(vals[0] != Objs.Sym && vals[0].attr);
                                             ~~~~


Found 1 error in test.ts:7

</issue_description>

<agent_instructions>Write a test that shows the behavior first, commit that, then work on the issue.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Unique symbol defined as attribute not narrowed #2902

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