From b330fbce4deeab765ac3fa3b9ad07db282b92e27 Mon Sep 17 00:00:00 2001
From: Jake Bailey <5341706+jakebailey@users.noreply.github.com>
Date: Sat, 30 Sep 2023 00:54:10 -0700
Subject: [PATCH 1/2] Bail early in getNarrowedTypeWorker if type is candidate

---
 src/compiler/checker.ts | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 58d6a8b867e20..3c817e95708eb 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -28163,6 +28163,12 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
             if (type.flags & TypeFlags.AnyOrUnknown) {
                 return candidate;
             }
+
+            // TODO(jakebailey): should this just be type === candidate?
+            if (isTypeIdenticalTo(type, candidate)) {
+                return candidate;
+            }
+
             // We first attempt to filter the current type, narrowing constituents as appropriate and removing
             // constituents that are unrelated to the candidate.
             const isRelated = checkDerived ? isTypeDerivedFrom : isTypeSubtypeOf;

From 1c14f5cfbe603954e7d804c12335faf58216cd72 Mon Sep 17 00:00:00 2001
From: Jake Bailey <5341706+jakebailey@users.noreply.github.com>
Date: Sun, 1 Oct 2023 20:36:31 -0700
Subject: [PATCH 2/2] And in negative case, use equality

---
 src/compiler/checker.ts | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 3c817e95708eb..8e5a04d629c7f 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -28154,6 +28154,9 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
 
         function getNarrowedTypeWorker(type: Type, candidate: Type, assumeTrue: boolean, checkDerived: boolean) {
             if (!assumeTrue) {
+                if (type === candidate) {
+                    return neverType;
+                }
                 if (checkDerived) {
                     return filterType(type, t => !isTypeDerivedFrom(t, candidate));
                 }
@@ -28163,9 +28166,7 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
             if (type.flags & TypeFlags.AnyOrUnknown) {
                 return candidate;
             }
-
-            // TODO(jakebailey): should this just be type === candidate?
-            if (isTypeIdenticalTo(type, candidate)) {
+            if (type === candidate) {
                 return candidate;
             }