[Repo Assist] [JS/TS/Python/Beam] Fix ResizeArray equality to use reference equality (fixes #3718) (#4524)

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Co-authored-by: Dag Brattli <dag@brattli.net>
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: 4 people

src/Fable.Cli/CHANGELOG.md

@@ -9,6 +9,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 
+* [JS/TS/Python/Beam] Fix `ResizeArray` (`System.Collections.Generic.List`) equality to use reference equality instead of structural equality (fixes #3718)
 * [Beam] Fix `List.Cons` call replacement and test (by @ncave)
 * [Beam/Dart/Python/TypeScript] Fix `Array.Equals` to use reference equality instead of structural equality (by @ncave)
 * [Dart/Python/TypeScript/Rust] Fix `Seq.foldBack2` for sequences with different lengths (by @ncave)

src/Fable.Compiler/CHANGELOG.md

@@ -9,6 +9,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 
+* [JS/TS/Python/Beam] Fix `ResizeArray` (`System.Collections.Generic.List`) equality to use reference equality instead of structural equality (fixes #3718)
 * [Beam] Fix `List.Cons` call replacement and test (by @ncave)
 * [Beam/Dart/Python/TypeScript] Fix `Array.Equals` to use reference equality instead of structural equality (by @ncave)
 * [Dart/Python/TypeScript/Rust] Fix `Seq.foldBack2` for sequences with different lengths (by @ncave)

src/Fable.Transforms/Beam/Replacements.fs

@@ -11,12 +11,19 @@ type Context = FSharp2Fable.Context
 type ICompiler = FSharp2Fable.IFableCompiler
 type CallInfo = ReplaceCallInfo
 
+let private physicalEquals r (left: Expr) (right: Expr) =
+    emitExpr r Boolean [ left; right ] "($0 =:= $1)"
+
 // Use fable_comparison:equals/2 for structural equality.
 // This handles ref-wrapped arrays (process dict refs) by dereferencing
 // before comparison, so structural equality works correctly.
 let private equals (com: ICompiler) r equal (left: Expr) (right: Expr) =
     let eqCall =
-        Helper.LibCall(com, "fable_comparison", "equals", Boolean, [ left; right ], ?loc = r)
+        match left.Type with
+        // ResizeArray (System.Collections.Generic.List) uses reference equality in .NET, see #3718.
+        // Two distinct refs are never =:=, so this matches .NET semantics directly.
+        | Array(_, ResizeArray) -> physicalEquals r left right
+        | _ -> Helper.LibCall(com, "fable_comparison", "equals", Boolean, [ left; right ], ?loc = r)
 
     if equal then
         eqCall
@@ -26,9 +33,6 @@ let private equals (com: ICompiler) r equal (left: Expr) (right: Expr) =
 let private compare (com: ICompiler) r (left: Expr) (right: Expr) =
     Helper.LibCall(com, "fable_comparison", "compare", Number(Int32, NumberInfo.Empty), [ left; right ], ?loc = r)
 
-let private physicalEquals r (left: Expr) (right: Expr) =
-    emitExpr r Boolean [ left; right ] "($0 =:= $1)"
-
 /// Deref an array ref to its underlying list (for passing to list BIFs).
 /// Byte arrays (UInt8) are atomics — convert to list via fable_utils:byte_array_to_list.
 let private derefArr r (expr: Expr) =

src/Fable.Transforms/Python/Replacements.fs

@@ -620,6 +620,16 @@ let rec equals (com: ICompiler) ctx r equal (left: Expr) (right: Expr) =
     | DeclaredType _ ->
         Helper.LibCall(com, "util", "equals", Boolean, [ left; right ], ?loc = r)
         |> is equal
+    | Array(_, ResizeArray) ->
+        // ResizeArray (System.Collections.Generic.List) uses reference equality in .NET, see #3718
+        makeEqOpStrict
+            r
+            left
+            right
+            (if equal then
+                 BinaryEqual
+             else
+                 BinaryUnequal)
     | Array(t, _) ->
         let f = makeEqualityFunction com ctx t
 

src/Fable.Transforms/Replacements.fs

@@ -566,6 +566,12 @@ let rec equals (com: ICompiler) ctx r equal (left: Expr) (right: Expr) =
     | DeclaredType _ ->
         Helper.LibCall(com, "Util", "equals", Boolean, [ left; right ], ?loc = r)
         |> is equal
+    | Array(_, ResizeArray) ->
+        // ResizeArray (System.Collections.Generic.List) uses reference equality in .NET, see #3718
+        if equal then
+            makeBinOp r Boolean left right BinaryEqual
+        else
+            makeBinOp r Boolean left right BinaryUnequal
     | Array(t, _) ->
         let f = makeEqualityFunction com ctx t
 

src/fable-library-ts/CHANGELOG.md

@@ -9,6 +9,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 
+* [JS/TS] Fix `ResizeArray` (`System.Collections.Generic.List`) equality to use reference equality instead of structural equality (fixes #3718)
 * [JS/TS] Fix `String.Contains` ignoring `StringComparison` argument (second argument was silently discarded)
 
 ## 2.0.0-rc.6 - 2026-04-07

tests/Beam/ResizeArrayTests.fs

@@ -348,3 +348,12 @@ let ``test ResizeArray works with Seq.item`` () =
     equal 2 secondItem
     let thirdItem = Seq.item 2 li
     equal 3 thirdItem
+
+[<Fact>]
+let ``test ResizeArray uses reference equality not structural equality`` () = // See #3718
+    let xs = ResizeArray [0; 1; 2]
+    let ys = ResizeArray [0; 1; 2]
+    equal false (xs = ys)
+    equal true (xs = xs)
+    equal true (xs <> ys)
+    equal false (xs <> xs)

tests/Js/Main/ResizeArrayTests.fs

@@ -376,4 +376,12 @@ let tests =
         let xs = ResizeArray [1; 2; 3]
         throwsAnyError (fun () -> xs.[-1] <- 42)
         throwsAnyError (fun () -> xs.[10] <- 42)
+
+    testCase "ResizeArray uses reference equality not structural equality" <| fun _ -> // See #3718
+        let xs = ResizeArray [0; 1; 2]
+        let ys = ResizeArray [0; 1; 2]
+        equal false (xs = ys)
+        equal true (xs = xs)
+        equal true (xs <> ys)
+        equal false (xs <> xs)
   ]

tests/Python/TestResizeArray.fs

@@ -347,4 +347,13 @@ let ``test ResizeArray works with Seq.item`` () =
     let secondItem = Seq.item 1 li
     equal 2 secondItem
     let thirdItem = Seq.item 2 li
-    equal 3 thirdItem
+    equal 3 thirdItem
+
+[<Fact>]
+let ``test ResizeArray uses reference equality not structural equality`` () = // See #3718
+    let xs = ResizeArray [0; 1; 2]
+    let ys = ResizeArray [0; 1; 2]
+    equal false (xs = ys)
+    equal true (xs = xs)
+    equal true (xs <> ys)
+    equal false (xs <> xs)