From d62be82778211ad0edee565d848205ee739d8efb Mon Sep 17 00:00:00 2001
From: Robert de Forest <robert@defore.st>
Date: Wed, 3 Jun 2026 00:19:37 -0700
Subject: [PATCH 1/2] Fixes #5480: clear  when re-inverting an 'in' Op

---
 lib/coffeescript/nodes.js |  6 +++++-
 src/nodes.coffee          |  5 ++++-
 test/operators.coffee     | 23 +++++++++++++++++++++++
 3 files changed, 32 insertions(+), 2 deletions(-)

diff --git a/lib/coffeescript/nodes.js b/lib/coffeescript/nodes.js
index 3b9bc9b544..fc1c0d96b6 100644
--- a/lib/coffeescript/nodes.js
+++ b/lib/coffeescript/nodes.js
@@ -6987,7 +6987,11 @@
       invert() {
         var allInvertable, curr, fst, op, ref1;
         if (this.isInOperator()) {
-          this.invertOperator = '!';
+          if (this.invertOperator) {
+            this.invertOperator = void 0;
+          } else {
+            this.invertOperator = '!';
+          }
           return this;
         }
         if (this.isChain()) {
diff --git a/src/nodes.coffee b/src/nodes.coffee
index e38f85ae23..c380941720 100644
--- a/src/nodes.coffee
+++ b/src/nodes.coffee
@@ -4686,7 +4686,10 @@ exports.Op = class Op extends Base
 
   invert: ->
     if @isInOperator()
-      @invertOperator = '!'
+      if @invertOperator
+        @invertOperator = undefined
+      else
+        @invertOperator = '!'
       return @
     if @isChain()
       allInvertable = yes
diff --git a/test/operators.coffee b/test/operators.coffee
index aab8d03015..a1cdebfee9 100644
--- a/test/operators.coffee
+++ b/test/operators.coffee
@@ -451,3 +451,26 @@ test "'new' target", ->
 
   # classes must be called with `new`. In this case `new` applies to `get` only
   throws -> new get()()
+
+test "#5480: 'when a not in b'", ->
+  needle      = "needle"
+  hayStack    = []
+  needleStack = [needle]
+
+  switch
+    when needle     in    hayStack then ok false
+  switch
+    when needle not in    hayStack then ok true
+  switch
+    when needle     in needleStack then ok true
+  switch
+    when needle not in needleStack then ok false
+
+  switch "with subject"
+    when needle     in    hayStack then ok false
+  switch "with subject"
+    when needle not in    hayStack then ok true
+  switch "with subject"
+    when needle     in needleStack then ok true
+  switch "with subject"
+    when needle not in needleStack then ok false

From 4b48f357a4b6fa37407912e4f943c30ba5fcd81d Mon Sep 17 00:00:00 2001
From: Robert de Forest <robert@defore.st>
Date: Fri, 5 Jun 2026 23:23:58 -0700
Subject: [PATCH 2/2] test/operators: change switch subjects to boolean so they
 match the tests

---
 test/operators.coffee | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/test/operators.coffee b/test/operators.coffee
index a1cdebfee9..4910264c8f 100644
--- a/test/operators.coffee
+++ b/test/operators.coffee
@@ -466,11 +466,11 @@ test "#5480: 'when a not in b'", ->
   switch
     when needle not in needleStack then ok false
 
-  switch "with subject"
+  switch "with subject" and true
     when needle     in    hayStack then ok false
-  switch "with subject"
+  switch "with subject" and true
     when needle not in    hayStack then ok true
-  switch "with subject"
+  switch "with subject" and true
     when needle     in needleStack then ok true
-  switch "with subject"
+  switch "with subject" and true
     when needle not in needleStack then ok false