Bug 1599163 - fix nullish coalescing operator for (void 0) case; r=jorendorff

Differential Revision: https://phabricator.services.mozilla.com/D54775

--HG--
extra : moz-landing-system : lando

js/src/frontend/FoldConstants.cpp

@@ -718,6 +718,7 @@ static bool FoldAndOrCoalesce(JSContext* cx, ParseNode** nodePtr) {
 
     bool isTruthyCoalesceNode =
         isCoalesceNode && !((*elem)->isKind(ParseNodeKind::NullExpr) ||
+                            (*elem)->isKind(ParseNodeKind::VoidExpr) ||
                             (*elem)->isKind(ParseNodeKind::RawUndefinedExpr));
     bool canShortCircuit = (isOrNode && t == Truthy) ||
                            (isAndNode && t == Falsy) || isTruthyCoalesceNode;

js/src/tests/non262/expressions/nullish-coalescing.js

@@ -67,6 +67,7 @@ shouldBe(1 * null ?? 3, 0);
 shouldBe(1 / null ?? 3, Infinity);
 shouldBe(isNaN(1 % null ?? 3), true);
 shouldBe(1 ** null ?? 3, 1);
+shouldBe((void 0) ?? 3, 3);
 
 const obj = {
       count: 0,