From f7bb79d6d16588766fe42f032ca84048bf74bca2 Mon Sep 17 00:00:00 2001
From: Luke Wilde <lukew@serenityos.org>
Date: Sun, 16 Oct 2022 00:13:44 +0100
Subject: [PATCH] LibJS: Fast-path ToTemporalTimeZone when the argument is a
 TimeZone

This is a normative change in the Temporal spec.

See: https://github.com/tc39/proposal-temporal/commit/54cea53
---
 .../LibJS/Runtime/Temporal/TimeZone.cpp        | 14 ++++++++++----
 .../Temporal/TimeZone/TimeZone.from.js         | 18 ++++++++++++++++++
 2 files changed, 28 insertions(+), 4 deletions(-)

diff --git a/Userland/Libraries/LibJS/Runtime/Temporal/TimeZone.cpp b/Userland/Libraries/LibJS/Runtime/Temporal/TimeZone.cpp
index ea46236c0b..eadf023472 100644
--- a/Userland/Libraries/LibJS/Runtime/Temporal/TimeZone.cpp
+++ b/Userland/Libraries/LibJS/Runtime/Temporal/TimeZone.cpp
@@ -292,7 +292,13 @@ ThrowCompletionOr<Object*> to_temporal_time_zone(VM& vm, Value temporal_time_zon
 {
     // 1. If Type(temporalTimeZoneLike) is Object, then
     if (temporal_time_zone_like.is_object()) {
-        // a. If temporalTimeZoneLike has an [[InitializedTemporalZonedDateTime]] internal slot, then
+        // a. If temporalTimeZoneLike has an [[InitializedTemporalTimeZone]] internal slot, then
+        if (is<TimeZone>(temporal_time_zone_like.as_object())) {
+            // i. Return temporalTimeZoneLike.
+            return &temporal_time_zone_like.as_object();
+        }
+
+        // b. If temporalTimeZoneLike has an [[InitializedTemporalZonedDateTime]] internal slot, then
         if (is<ZonedDateTime>(temporal_time_zone_like.as_object())) {
             auto& zoned_date_time = static_cast<ZonedDateTime&>(temporal_time_zone_like.as_object());
 
@@ -300,14 +306,14 @@ ThrowCompletionOr<Object*> to_temporal_time_zone(VM& vm, Value temporal_time_zon
             return &zoned_date_time.time_zone();
         }
 
-        // b. If ? HasProperty(temporalTimeZoneLike, "timeZone") is false, return temporalTimeZoneLike.
+        // c. If ? HasProperty(temporalTimeZoneLike, "timeZone") is false, return temporalTimeZoneLike.
         if (!TRY(temporal_time_zone_like.as_object().has_property(vm.names.timeZone)))
             return &temporal_time_zone_like.as_object();
 
-        // c. Set temporalTimeZoneLike to ? Get(temporalTimeZoneLike, "timeZone").
+        // d. Set temporalTimeZoneLike to ? Get(temporalTimeZoneLike, "timeZone").
         temporal_time_zone_like = TRY(temporal_time_zone_like.as_object().get(vm.names.timeZone));
 
-        // d. If Type(temporalTimeZoneLike) is Object and ? HasProperty(temporalTimeZoneLike, "timeZone") is false, return temporalTimeZoneLike.
+        // e. If Type(temporalTimeZoneLike) is Object and ? HasProperty(temporalTimeZoneLike, "timeZone") is false, return temporalTimeZoneLike.
         if (temporal_time_zone_like.is_object() && !TRY(temporal_time_zone_like.as_object().has_property(vm.names.timeZone)))
             return &temporal_time_zone_like.as_object();
     }
diff --git a/Userland/Libraries/LibJS/Tests/builtins/Temporal/TimeZone/TimeZone.from.js b/Userland/Libraries/LibJS/Tests/builtins/Temporal/TimeZone/TimeZone.from.js
index 357f1492b5..5e77c01cd7 100644
--- a/Userland/Libraries/LibJS/Tests/builtins/Temporal/TimeZone/TimeZone.from.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Temporal/TimeZone/TimeZone.from.js
@@ -45,4 +45,22 @@ describe("normal behavior", () => {
             expect(Temporal.TimeZone.from(arg).id).toBe(expected);
         }
     });
+
+    test("ToTemporalTimeZone fast path returns if it is passed a Temporal.TimeZone instance", () => {
+        // This is obseravble via there being no property lookups (avoiding a "timeZone" property lookup in this case)
+        let madeObservableHasPropertyLookup = false;
+        class TimeZone extends Temporal.TimeZone {
+            constructor() {
+                super("UTC");
+            }
+
+            get timeZone() {
+                madeObservableHasPropertyLookup = true;
+                return this;
+            }
+        }
+        const timeZone = new TimeZone();
+        Temporal.TimeZone.from(timeZone);
+        expect(madeObservableHasPropertyLookup).toBeFalse();
+    });
 });