From 69da8125d010b67c4e71509fa06cd80777bae0d9 Mon Sep 17 00:00:00 2001
From: Jesse Alama <jesse@igalia.com>
Date: Thu, 5 Mar 2026 15:55:12 +0100
Subject: [PATCH 1/3] Polyfill: Replace unreachable overshoot with assertion in
 calendarToIsoDate

---
 polyfill/lib/calendar.mjs | 24 ++++--------------------
 1 file changed, 4 insertions(+), 20 deletions(-)

diff --git a/polyfill/lib/calendar.mjs b/polyfill/lib/calendar.mjs
index 52a17254b..63de87011 100644
--- a/polyfill/lib/calendar.mjs
+++ b/polyfill/lib/calendar.mjs
@@ -987,7 +987,6 @@ const nonIsoHelperBase = {
     let increment = 8;
     while (sign) {
       isoEstimate = ES.AddDaysToISODate(isoEstimate, sign * increment);
-      const oldRoundtripEstimate = roundtripEstimate;
       roundtripEstimate = this.isoToCalendarDate(isoEstimate, cache);
       const oldSign = sign;
       sign = this.compareCalendarDates(date, roundtripEstimate);
@@ -998,25 +997,10 @@ const nonIsoHelperBase = {
           // Signal the loop condition that there's a match.
           sign = 0;
         } else if (oldSign && sign !== oldSign) {
-          if (increment > 1) {
-            // If the estimate overshot the target, try again with a smaller increment
-            // in the reverse direction.
-            increment /= 2;
-          } else {
-            // Increment is 1, and neither the previous estimate nor the new
-            // estimate is correct. The only way that can happen is if the
-            // original date was an invalid value that will be constrained or
-            // rejected here.
-            if (overflow === 'reject') {
-              throw new RangeErrorCtor(`Can't find ISO date from calendar date: ${JSONStringify({ ...originalDate })}`);
-            } else {
-              // To constrain, pick the earliest value
-              const order = this.compareCalendarDates(roundtripEstimate, oldRoundtripEstimate);
-              // If current value is larger, then back up to the previous value.
-              if (order > 0) isoEstimate = ES.AddDaysToISODate(isoEstimate, -1);
-              sign = 0;
-            }
-          }
+          /* c8 ignore next */
+          assertNotReached(
+            `calendarToIsoDate binary search should not overshoot (increment ${increment} cannot skip a month)`
+          );
         }
       }
     }

From 501d4ae1c7f636c103d1e02449e3d4f35f114997 Mon Sep 17 00:00:00 2001
From: Philip Chimento <pchimento@igalia.com>
Date: Thu, 5 Mar 2026 12:15:03 -0800
Subject: [PATCH 2/3] (fixup) Convert assertNotReached into assert

---
 polyfill/lib/calendar.mjs | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/polyfill/lib/calendar.mjs b/polyfill/lib/calendar.mjs
index 63de87011..b9af42fb3 100644
--- a/polyfill/lib/calendar.mjs
+++ b/polyfill/lib/calendar.mjs
@@ -996,9 +996,9 @@ const nonIsoHelperBase = {
           isoEstimate = calculateSameMonthResult(diff.days);
           // Signal the loop condition that there's a match.
           sign = 0;
-        } else if (oldSign && sign !== oldSign) {
-          /* c8 ignore next */
-          assertNotReached(
+        } else {
+          assert(
+            !oldSign || sign === oldSign,
             `calendarToIsoDate binary search should not overshoot (increment ${increment} cannot skip a month)`
           );
         }

From faf750a8c21366f89e64f221716a146c26045e19 Mon Sep 17 00:00:00 2001
From: Philip Chimento <pchimento@igalia.com>
Date: Thu, 5 Mar 2026 12:29:22 -0800
Subject: [PATCH 3/3] (fixup) Simplify and update comments

---
 polyfill/lib/calendar.mjs | 27 +++++++++++----------------
 1 file changed, 11 insertions(+), 16 deletions(-)

diff --git a/polyfill/lib/calendar.mjs b/polyfill/lib/calendar.mjs
index b9af42fb3..d50ece6df 100644
--- a/polyfill/lib/calendar.mjs
+++ b/polyfill/lib/calendar.mjs
@@ -982,27 +982,22 @@ const nonIsoHelperBase = {
         sign = this.compareCalendarDates(date, roundtripEstimate);
       }
     }
-    // If the initial guess is not in the same month, then bisect the
-    // distance to the target, starting with 8 days per step.
-    let increment = 8;
+    // If the initial guess is not in the same month, search in 8-day increments
+    // towards the target, until we get into the same month. From there we can
+    // calculate the number of days directly.
+    const increment = 8;
+    const oldSign = sign;
     while (sign) {
       isoEstimate = ES.AddDaysToISODate(isoEstimate, sign * increment);
       roundtripEstimate = this.isoToCalendarDate(isoEstimate, cache);
-      const oldSign = sign;
       sign = this.compareCalendarDates(date, roundtripEstimate);
-      if (sign) {
-        diff = simpleDateDiff(date, roundtripEstimate);
-        if (diff.years === 0 && diff.months === 0) {
-          isoEstimate = calculateSameMonthResult(diff.days);
-          // Signal the loop condition that there's a match.
-          sign = 0;
-        } else {
-          assert(
-            !oldSign || sign === oldSign,
-            `calendarToIsoDate binary search should not overshoot (increment ${increment} cannot skip a month)`
-          );
-        }
+      if (sign === 0) break;
+      diff = simpleDateDiff(date, roundtripEstimate);
+      if (diff.years === 0 && diff.months === 0) {
+        isoEstimate = calculateSameMonthResult(diff.days);
+        break;
       }
+      assert(sign === oldSign, 'calendarToIsoDate search should not overshoot a month entirely');
     }
     cache.set(key, isoEstimate);
     if (keyOriginal) cache.set(keyOriginal, isoEstimate);