[JS:Engine] Implement support for nullish coalescing in the JS parser

- UXP Parts 1, 2, 4, 5, and 7 based on Bug 1566141
- UXP Part 3 partially based on Bugs 1566141 and 1593415
- UXP Part 6 partly based on Bugs 1566141 and 1599163 with a different approach by modifying the `Boolish` function directly to act differently if we're checking for nullish values.

1 files changed, 91 insertions, 0 deletions

diff --git a/js/src/tests/test262/language/expressions/coalesce/short-circuit-number-object.js b/js/src/tests/test262/language/expressions/coalesce/short-circuit-number-object.js
new file mode 100644
index 000000000..5f2eee11e
--- /dev/null
+++ b/js/src/tests/test262/language/expressions/coalesce/short-circuit-number-object.js
@@ -0,0 +1,91 @@
+// Copyright (C) 2019 Leo Balter. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+description: >
+    Short circuit if the CoalesceExpressionHead is not undefined or null (object)
+esid: sec-conditional-operator
+info: |
+    ConditionalExpression :
+        ShortCircuitExpression
+        ShortCircuitExpression ? AssignmentExpression : AssignmentExpression
+
+    ShortCircuitExpression :
+        LogicalORExpression
+        CoalesceExpression
+
+    CoalesceExpression :
+        CoalesceExpressionHead ?? BitwiseORExpression
+
+    CoalesceExpressionHead :
+        CoalesceExpression
+        BitwiseORExpression
+
+    Runtime Semantics: Evaluation
+
+    CoalesceExpression:CoalesceExpressionHead??BitwiseORExpression
+
+    1. Let lref be the result of evaluating CoalesceExpressionHead.
+    2. Let lval be ? GetValue(lref).
+    3. If lval is undefined or null,
+        a. Let rref be the result of evaluating BitwiseORExpression.
+        b. Return ? GetValue(rref).
+    4. Otherwise, return lval.
+features: [coalesce-expression]
+---*/
+
+var x;
+var obj = {
+    toString() {
+        return null;
+    },
+    valueOf() {
+        return null;
+    }
+};
+
+x = undefined;
+x = obj ?? 1;
+assert.sameValue(x, obj, 'obj ?? 1');
+
+x = undefined;
+x = obj ?? null;
+assert.sameValue(x, obj, 'obj ?? null');
+
+x = undefined;
+x = obj ?? undefined;
+assert.sameValue(x, obj, 'obj ?? undefined');
+
+x = undefined;
+x = obj ?? null ?? undefined;
+assert.sameValue(x, obj, 'obj ?? null ?? undefined');
+
+x = undefined;
+x = obj ?? undefined ?? null;
+assert.sameValue(x, obj, 'obj ?? undefined ?? null');
+
+x = undefined;
+x = obj ?? null ?? null;
+assert.sameValue(x, obj, 'obj ?? null ?? null');
+
+x = undefined;
+x = obj ?? undefined ?? undefined;
+assert.sameValue(x, obj, 'obj ?? null ?? null');
+
+x = undefined;
+x = null ?? obj ?? null;
+assert.sameValue(x, obj, 'null ?? obj ?? null');
+
+x = undefined;
+x = null ?? obj ?? undefined;
+assert.sameValue(x, obj, 'null ?? obj ?? undefined');
+
+x = undefined;
+x = undefined ?? obj ?? null;
+assert.sameValue(x, obj, 'undefined ?? obj ?? null');
+
+x = undefined;
+x = undefined ?? obj ?? undefined;
+assert.sameValue(x, obj, 'undefined ?? obj ?? undefined');
+
+reportCompare(0, 0);