MangelMaxime · Mar 20, 2026 · Mar 20, 2026 · Mar 20, 2026 · Mar 20, 2026
diff --git a/src/Fable.Cli/CHANGELOG.md b/src/Fable.Cli/CHANGELOG.md
@@ -7,6 +7,13 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## Unreleased
 
+### Fixed
+
+* [JS/TS] Fix `Unchecked.defaultof<'T>` for struct types with fields returning `undefined` instead of zero-initialized values (by @MangelMaxime)
+* [Python] Fix `Unchecked.defaultof<'T>` for struct types with fields returning incorrect values instead of zero-initialized instances (by @MangelMaxime)
+* [Python] Fix `Unchecked.defaultof<char>` returning `""` (empty string) instead of `"\u0000"` (null character) (by @MangelMaxime)
+* [Python] Improve `Unchecked.defaultof<_>` for declared entities (by @MangelMaxime)
+
 ## 5.0.0-rc.5 - 2026-03-20
 
 ### Added

diff --git a/src/Fable.Compiler/CHANGELOG.md b/src/Fable.Compiler/CHANGELOG.md
@@ -7,6 +7,13 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## Unreleased
 
+### Fixed
+
+* [JS/TS] Fix `Unchecked.defaultof<'T>` for struct types with fields returning `undefined` instead of zero-initialized values (by @MangelMaxime)
+* [Python] Fix `Unchecked.defaultof<'T>` for struct types with fields returning incorrect values instead of zero-initialized instances (by @MangelMaxime)
+* [Python] Fix `Unchecked.defaultof<char>` returning `""` (empty string) instead of `"\u0000"` (null character) (by @MangelMaxime)
+* [Python] Improve `Unchecked.defaultof<_>` for declared entities (by @MangelMaxime)
+
 ## 5.0.0-rc.11 - 2026-03-20
 
 ### Added

diff --git a/src/Fable.Transforms/Python/Fable2Python.Util.fs b/src/Fable.Transforms/Python/Fable2Python.Util.fs
@@ -805,9 +805,15 @@ module Util =
             | Float32 -> makeFloat com ctx None t "float32" 0.0 |> fst
             | Float64 -> makeFloat com ctx None t "float64" 0.0 |> fst
             | Decimal -> makeFloat com ctx None t "float64" 0.0 |> fst
-        | Fable.String
-        | Fable.Char -> Expression.stringConstant ""
-        | Fable.DeclaredType(ent, _) -> Expression.none
+        | Fable.Char -> Expression.stringConstant "\u0000"
+        | Fable.String -> Expression.stringConstant ""
+        | Fable.DeclaredType(entRef, _) ->
+            match com.TryGetEntity(entRef) with
+            | Some ent when ent.IsValueType ->
+                match tryPyConstructor com ctx ent with
+                | Some(ctorExpr, _stmts) -> Expression.call (ctorExpr, [])
+                | None -> Expression.none
+            | _ -> Expression.none
         | Fable.GenericParam _ -> libValue com ctx "util" "UNIT"
         | _ -> Expression.none
 

diff --git a/src/Fable.Transforms/Python/Replacements.fs b/src/Fable.Transforms/Python/Replacements.fs
@@ -727,20 +727,93 @@ let makeHashSet (com: ICompiler) ctx r t sourceSeq =
         makeEqualityComparer com ctx key |> makeHashSetWithComparer com r t sourceSeq
     | _ -> Helper.GlobalCall("set", t, [ sourceSeq ], isConstructor = true, ?loc = r)
 
-let rec getZero (com: ICompiler) ctx (t: Type) =
+let tryEntityIdent (com: Compiler) entFullName =
+    match entFullName with
+    | BuiltinDefinition BclDateOnly
+    | BuiltinDefinition BclDateTime
+    | BuiltinDefinition BclDateTimeOffset -> makeIdentExpr "Date" |> Some
+    | BuiltinDefinition BclTimer -> makeImportLib com Any "default" "Timer" |> Some
+    | BuiltinDefinition(FSharpReference _) -> makeImportLib com Any "FSharpRef" "Types" |> Some
+    | BuiltinDefinition(FSharpResult _) ->
+        // Import the underscore-prefixed base class (has cases() method), not the type alias
+        makeImportLib com Any "_FSharpResult_2" "Result" |> Some
+    | BuiltinDefinition(FSharpChoice genArgs) ->
+        // Import the underscore-prefixed base class (has cases() method), not the type alias
+        let membName = $"_FSharpChoice_%d{List.length genArgs}"
+        makeImportLib com Any membName "Choice" |> Some
+    // | BuiltinDefinition BclGuid -> jsTypeof "string" expr
+    // | BuiltinDefinition BclTimeSpan -> jsTypeof "number" expr
+    // | BuiltinDefinition BclHashSet _ -> fail "MutableSet" // TODO:
+    // | BuiltinDefinition BclDictionary _ -> fail "MutableMap" // TODO:
+    // | BuiltinDefinition BclKeyValuePair _ -> fail "KeyValuePair" // TODO:
+    // | BuiltinDefinition FSharpSet _ -> fail "Set" // TODO:
+    // | BuiltinDefinition FSharpMap _ -> fail "Map" // TODO:
+    | Types.matchFail -> makeImportLib com Any "MatchFailureException" "Types" |> Some
+    | Types.exception_ -> makeIdentExpr "Exception" |> Some
+    // | "System.Collections.Generic.KeyNotFoundException" ->
+    //     makeImportLib com Any "KeyNotFoundException" "System.Collections.Generic" |> Some
+    | BuiltinSystemException entName -> makeImportLib com Any entName "System" |> Some
+    // | Naming.EndsWith "Exception" _ -> makeIdentExpr "Exception" |> Some
+    | _ -> None
+
+let tryConstructor com (ent: Entity) =
+    if FSharp2Fable.Util.isReplacementCandidate ent.Ref then
+        tryEntityIdent com (ent.FullName |> Naming.toPythonNaming)
+    else
+        match FSharp2Fable.Util.tryEntityIdentMaybeGlobalOrImported com ent with
+        | Some(IdentExpr ident) when ent.IsFSharpUnion ->
+            // For F# union types, the base class is prefixed with underscore (_UnionName)
+            // This is needed for both reflection (base class has cases() method) and
+            // type annotations (self inside base class methods)
+            Some(IdentExpr { ident with Name = "_" + ident.Name })
+        | other -> other
+
+let constructor com ent =
+    match tryConstructor com ent with
+    | Some e -> e
+    | None -> $"Cannot find %s{ent.FullName} constructor" |> addErrorAndReturnNull com [] None
+
+/// For user-defined value types (structs), returns a constructor call initialized with
+/// a zero value for each instance field. Only instance fields are constructor parameters;
+/// static fields (from `static let` bindings) are excluded to avoid deep recursion through
+/// static field type graphs. Returns None if no constructor is available or the type is
+/// not a value type.
+let rec private tryZeroValueTypeConstructor (com: ICompiler) (ctx: Context) (t: Type) (ent: Entity) =
+    if ent.IsValueType then
+        tryConstructor com ent
+        |> Option.map (fun e ->
+            let args =
+                ent.FSharpFields
+                |> List.choose (fun f ->
+                    if f.IsStatic then
+                        None
+                    else
+                        getZero com ctx f.FieldType |> Some
+                )
+
+            Helper.ConstructorCall(e, t, args)
+        )
+    else
+        None
+
+and private getZero (com: ICompiler) ctx (t: Type) =
     match t with
     | Boolean -> makeBoolConst false
     | Number(BigInt, _) as t -> Helper.LibCall(com, "big_int", "fromInt32", t, [ makeIntConst 0 ])
     | Number(Decimal, _) as t -> makeIntConst 0 |> makeDecimalFromExpr com None t
     | Number(kind, uom) -> NumberConstant(NumberValue.GetZero kind, uom) |> makeValue None
-    | Char
+    | Char -> makeCharConst '\u0000'
     | String -> makeStrConst "" // TODO: Use null for string?
     | Builtin BclTimeSpan -> Helper.LibCall(com, "time_span", "create", t, [ makeIntConst 0 ])
     | Builtin BclDateTime as t -> Helper.LibCall(com, "date", "minValue", t, [])
     | Builtin BclDateTimeOffset as t -> Helper.LibCall(com, "DateOffset", "minValue", t, [])
     | Builtin(FSharpSet genArg) as t -> makeSet com ctx None t "Empty" [] genArg
     | Builtin(BclKeyValuePair(k, v)) -> makeTuple None true [ getZero com ctx k; getZero com ctx v ]
     | ListSingleton(CustomOp com ctx None t "get_Zero" [] e) -> e
+    | DeclaredType(entRef, _) ->
+        com.GetEntity(entRef)
+        |> tryZeroValueTypeConstructor com ctx t
+        |> Option.defaultValue (Value(Null Any, None)) // null
     | _ -> Value(Null Any, None) // null
 
 let getOne (com: ICompiler) ctx (t: Type) =
@@ -810,52 +883,6 @@ let injectArg (com: ICompiler) (ctx: Context) r moduleName methName (genArgs: Ty
         | None -> args
         | Some injectInfo -> injectArgInner args injectInfo
 
-let tryEntityIdent (com: Compiler) entFullName =
-    match entFullName with
-    | BuiltinDefinition BclDateOnly
-    | BuiltinDefinition BclDateTime
-    | BuiltinDefinition BclDateTimeOffset -> makeIdentExpr "Date" |> Some
-    | BuiltinDefinition BclTimer -> makeImportLib com Any "default" "Timer" |> Some
-    | BuiltinDefinition(FSharpReference _) -> makeImportLib com Any "FSharpRef" "Types" |> Some
-    | BuiltinDefinition(FSharpResult _) ->
-        // Import the underscore-prefixed base class (has cases() method), not the type alias
-        makeImportLib com Any "_FSharpResult_2" "Result" |> Some
-    | BuiltinDefinition(FSharpChoice genArgs) ->
-        // Import the underscore-prefixed base class (has cases() method), not the type alias
-        let membName = $"_FSharpChoice_%d{List.length genArgs}"
-        makeImportLib com Any membName "Choice" |> Some
-    // | BuiltinDefinition BclGuid -> jsTypeof "string" expr
-    // | BuiltinDefinition BclTimeSpan -> jsTypeof "number" expr
-    // | BuiltinDefinition BclHashSet _ -> fail "MutableSet" // TODO:
-    // | BuiltinDefinition BclDictionary _ -> fail "MutableMap" // TODO:
-    // | BuiltinDefinition BclKeyValuePair _ -> fail "KeyValuePair" // TODO:
-    // | BuiltinDefinition FSharpSet _ -> fail "Set" // TODO:
-    // | BuiltinDefinition FSharpMap _ -> fail "Map" // TODO:
-    | Types.matchFail -> makeImportLib com Any "MatchFailureException" "Types" |> Some
-    | Types.exception_ -> makeIdentExpr "Exception" |> Some
-    // | "System.Collections.Generic.KeyNotFoundException" ->
-    //     makeImportLib com Any "KeyNotFoundException" "System.Collections.Generic" |> Some
-    | BuiltinSystemException entName -> makeImportLib com Any entName "System" |> Some
-    // | Naming.EndsWith "Exception" _ -> makeIdentExpr "Exception" |> Some
-    | _ -> None
-
-let tryConstructor com (ent: Entity) =
-    if FSharp2Fable.Util.isReplacementCandidate ent.Ref then
-        tryEntityIdent com (ent.FullName |> Naming.toPythonNaming)
-    else
-        match FSharp2Fable.Util.tryEntityIdentMaybeGlobalOrImported com ent with
-        | Some(IdentExpr ident) when ent.IsFSharpUnion ->
-            // For F# union types, the base class is prefixed with underscore (_UnionName)
-            // This is needed for both reflection (base class has cases() method) and
-            // type annotations (self inside base class methods)
-            Some(IdentExpr { ident with Name = "_" + ident.Name })
-        | other -> other
-
-let constructor com ent =
-    match tryConstructor com ent with
-    | Some e -> e
-    | None -> $"Cannot find %s{ent.FullName} constructor" |> addErrorAndReturnNull com [] None
-
 let tryOp com r t op args =
     Helper.LibCall(com, "option", "tryOp", t, op :: args, ?loc = r)
 
@@ -871,6 +898,7 @@ let rec defaultof com ctx r t =
     | Nullable _ -> Value(Null t, r)
     | Tuple(args, true) -> NewTuple(args |> List.map (defaultof com ctx r), true) |> makeValue None
     | Boolean
+    | Char
     | Number _
     | Builtin BclTimeSpan
     | Builtin BclDateTime
@@ -879,11 +907,7 @@ let rec defaultof com ctx r t =
     | DeclaredType(ent, _) ->
         let ent = com.GetEntity(ent)
         // TODO: For BCL types we cannot access the constructor, raise error or warning?
-        if ent.IsValueType then
-            tryConstructor com ent
-        else
-            None
-        |> Option.map (fun e -> Helper.ConstructorCall(e, t, []))
+        tryZeroValueTypeConstructor com ctx t ent
         |> Option.defaultWith (fun () -> Null t |> makeValue None)
     // TODO: Fail (or raise warning) if this is an unresolved generic parameter?
     | _ -> Null t |> makeValue None

diff --git a/src/Fable.Transforms/Replacements.fs b/src/Fable.Transforms/Replacements.fs
@@ -720,7 +720,30 @@ let constructor com ent =
     | Some e -> e
     | None -> $"Cannot find %s{ent.FullName} constructor" |> addErrorAndReturnNull com [] None
 
-let rec private getZero (com: ICompiler) (ctx: Context) (t: Type) =
+/// For user-defined value types (structs), returns a constructor call initialized with
+/// a zero value for each instance field. Only instance fields are constructor parameters;
+/// static fields (from `static let` bindings) are excluded to avoid deep recursion through
+/// static field type graphs. Returns None if no constructor is available or the type is
+/// not a value type.
+let rec private tryZeroValueTypeConstructor (com: ICompiler) (ctx: Context) (t: Type) (ent: Entity) =
+    if ent.IsValueType then
+        tryConstructor com ent
+        |> Option.map (fun e ->
+            let args =
+                ent.FSharpFields
+                |> List.choose (fun f ->
+                    if f.IsStatic then
+                        None
+                    else
+                        getZero com ctx f.FieldType |> Some
+                )
+
+            Helper.ConstructorCall(e, t, args)
+        )
+    else
+        None
+
+and private getZero (com: ICompiler) (ctx: Context) (t: Type) =
     match t with
     | Boolean -> makeBoolConst false
     | Char -> makeCharConst '\000'
@@ -734,29 +757,9 @@ let rec private getZero (com: ICompiler) (ctx: Context) (t: Type) =
     | Builtin(BclKeyValuePair(k, v)) -> makeTuple None true [ getZero com ctx k; getZero com ctx v ]
     | ListSingleton(CustomOp com ctx None t "get_Zero" [] e) -> e
     | DeclaredType(entRef, _) ->
-        let ent = com.GetEntity(entRef)
-        // For user-defined value types (structs), call the constructor with zero values
-        // for each instance field so Array.zeroCreate produces properly initialized instances
-        // instead of null. Only instance fields are constructor parameters; static fields
-        // (from `static let` bindings) are not, and iterating them could cause deep recursion
-        // through static field type graphs.
-        if ent.IsValueType then
-            tryConstructor com ent
-            |> Option.map (fun e ->
-                let args =
-                    ent.FSharpFields
-                    |> List.choose (fun f ->
-                        if f.IsStatic then
-                            None
-                        else
-                            getZero com ctx f.FieldType |> Some
-                    )
-
-                Helper.ConstructorCall(e, t, args)
-            )
-            |> Option.defaultValue (Value(Null Any, None)) // null
-        else
-            Value(Null Any, None) // null
+        com.GetEntity(entRef)
+        |> tryZeroValueTypeConstructor com ctx t
+        |> Option.defaultValue (Value(Null Any, None)) // null
     | _ -> Value(Null Any, None) // null
 
 let getOne (com: ICompiler) (ctx: Context) (t: Type) =
@@ -1008,11 +1011,7 @@ let rec defaultof (com: ICompiler) (ctx: Context) r t =
     | DeclaredType(entRef, _) ->
         let ent = com.GetEntity(entRef)
         // TODO: For BCL types we cannot access the constructor, raise error or warning?
-        if ent.IsValueType then
-            tryConstructor com ent
-        else
-            None
-        |> Option.map (fun e -> Helper.ConstructorCall(e, t, []))
+        tryZeroValueTypeConstructor com ctx t ent
         |> Option.defaultWith (fun () ->
             // Null t |> makeValue None
             Helper.LibCall(com, "Util", "defaultOf", t, [], ?loc = r)

diff --git a/tests/Js/Main/TypeTests.fs b/tests/Js/Main/TypeTests.fs
@@ -429,6 +429,21 @@ type ClassWithDefaultValueStaticFields() =
     static member Decimal = ClassWithDefaultValueStaticFields.decimal
     static member StructTuple2 = ClassWithDefaultValueStaticFields.structTuple2
 
+[<Struct>]
+type NestedStruct =
+
+    val A : float
+    val B : bool
+
+    new (a, b) = { A = a; B = b }
+
+[<Struct>]
+type TopLevelStruct =
+
+    val A : float
+    val B : NestedStruct
+
+    new (a, b) = { A = a; B = b }
 
 type Default1 = int
 
@@ -1600,4 +1615,10 @@ let tests =
         ClassWithDefaultValueStaticFields.Decimal |> equal 0M
 
         ClassWithDefaultValueStaticFields.IncrCount() |> equal 1
+
+    testCase "Unchecked.defaultof works for fields on structs" <| fun () ->
+        let top = TopLevelStruct()
+        top.A |> equal 0
+        top.B.A |> equal 0
+        top.B.B |> equal false
   ]
diff --git a/tests/Python/TestMisc.fs b/tests/Python/TestMisc.fs
@@ -1097,6 +1097,7 @@
     Unchecked.defaultof<int64> |> equal 0L
     Unchecked.defaultof<bigint> |> equal 0I
     Unchecked.defaultof<decimal> |> equal 0M
+    Unchecked.defaultof<char> |> equal '\u0000'
     Unchecked.defaultof<DateTime> |> equal DateTime.MinValue
     Unchecked.defaultof<TimeSpan> |> equal (TimeSpan.FromMilliseconds 0.)
     Unchecked.defaultof<bool> |> equal false
@@ -1137,7 +1138,7 @@
    | 2 -> x <- "2"
    | 3 | 4 -> x <- "3" // Multiple cases are allowed
    // | 5 | 6 as j -> x <- string j // This prevents the optimization
    | 4 -> x <- "4" // Unreachable cases are removed
    | _ -> x <- "?"
    equal "3" x


diff --git a/tests/Python/TestType.fs b/tests/Python/TestType.fs
@@ -362,6 +362,18 @@
     [<DefaultValue>]
     val mutable ObjValue: System.Collections.Generic.Dictionary<string, string>
 
+[<Struct>]
+type NestedStruct =
+    val A: float
+    val B: bool
+    new(a, b) = { A = a; B = b }
+
+[<Struct>]
+type TopLevelStruct =
+    val A: float
+    val B: NestedStruct
+    new(a, b) = { A = a; B = b }
+
 type Default1 = int
 
 type Distinct1 =
@@ -570,7 +582,7 @@

 type IndexedProps(v: int) =
    let mutable v = v
    member _.Item with get (v2: int) = v + v2 and set v2 (s: string) = v <- v2 + int s
    member _.Item with get (v2: float) = float v + v2 / 2.

 [<Interface>]
@@ -1757,3 +1769,10 @@
 
     let result3 = getTwoValues<TestTypeB, TestTypeC>()
     result3 |> equal ("B", "C")
+
+[<Fact>]
+let ``test Unchecked.defaultof works for fields on structs`` () =
+    let top = TopLevelStruct()
+    top.A |> equal 0
+    top.B.A |> equal 0
+    top.B.B |> equal false
diff --git a/tests/Rust/tests/src/MiscTests.fs b/tests/Rust/tests/src/MiscTests.fs
@@ -1196,6 +1196,7 @@
     Unchecked.defaultof<int64> |> equal 0L
     Unchecked.defaultof<bigint> |> equal 0I
     Unchecked.defaultof<decimal> |> equal 0M
+    Unchecked.defaultof<char> |> equal '\u0000'
     Unchecked.defaultof<DateTime> |> equal DateTime.MinValue
     Unchecked.defaultof<TimeSpan> |> equal (TimeSpan.FromMilliseconds 0.)
     Unchecked.defaultof<bool> |> equal false
@@ -1234,7 +1235,7 @@
    | 2 -> x <- "2"
    | 3 | 4 -> x <- "3" // Multiple cases are allowed
    // | 5 | 6 as j -> x <- string j // This prevents the optimization
    | 4 -> x <- "4" // Unreachable cases are removed
    | _ -> x <- "?"
    equal "3" x