Bugfix :: Object expressions in struct types generating invalid IL with byref fields (#19070)

Copilot · web-flow · commit a9d3eade1d29 · 2026-01-20T14:56:32.000+01:00
diff --git a/docs/release-notes/.FSharp.Compiler.Service/11.0.0.md b/docs/release-notes/.FSharp.Compiler.Service/11.0.0.md
@@ -14,6 +14,7 @@
 * Type relations cache: handle unsolved type variables ([Issue #19037](https://github.com/dotnet/fsharp/issues/19037)) ([PR #19040](https://github.com/dotnet/fsharp/pull/19040))
 * Fix insertion context for modules with multiline attributes. ([Issue #18671](https://github.com/dotnet/fsharp/issues/18671))
 * Fix `--typecheck-only` for scripts stopping after processing `#load`-ed script ([PR #19048](https://github.com/dotnet/fsharp/pull/19048))
+* Fix object expressions in struct types generating invalid IL with byref fields causing TypeLoadException at runtime. ([Issue #19068](https://github.com/dotnet/fsharp/issues/19068), [PR #19070](https://github.com/dotnet/fsharp/pull/19070))
 
 ### Added
 
diff --git a/src/Compiler/Checking/Expressions/CheckExpressions.fs b/src/Compiler/Checking/Expressions/CheckExpressions.fs
@@ -7236,6 +7236,13 @@ and TcObjectExpr (cenv: cenv) env tpenv (objTy, realObjTy, argopt, binds, extraI
     // Add the object type to the ungeneralizable items
     let env = {env with eUngeneralizableItems = addFreeItemOfTy objTy env.eUngeneralizableItems }
 
+    // Save the enclosing struct context BEFORE EnterFamilyRegion overwrites env.eFamilyType.
+    // This is used later to detect struct instance captures that would generate illegal byref fields.
+    let enclosingStructTyconRefOpt =
+        match env.eFamilyType with
+        | Some tcref when tcref.IsStructOrEnumTycon -> Some tcref
+        | _ -> None
+
     // Object expression members can access protected members of the implemented type
     let env = EnterFamilyRegion tcref env
     let ad = env.AccessRights
@@ -7344,8 +7351,20 @@ and TcObjectExpr (cenv: cenv) env tpenv (objTy, realObjTy, argopt, binds, extraI
            errorR (Error(FSComp.SR.tcInvalidObjectExpressionSyntaxForm (), mWholeExpr))
 
         // 4. Build the implementation
-        let expr = mkObjExpr(objtyR, baseValOpt, ctorCall, overrides', extraImpls, mWholeExpr)
-        let expr = mkCoerceIfNeeded g realObjTy objtyR expr
+        // Check for struct instance captures that would generate illegal byref fields.
+        // See AnalyzeObjExprStructCaptures and TransformObjExprForStructByrefCaptures for details.
+        let shouldTransform, structCaptures, _ = 
+            AnalyzeObjExprStructCaptures enclosingStructTyconRefOpt ctorCall overrides' extraImpls
+
+        let expr =
+            if not shouldTransform then
+                // No transformation needed - build the object expression directly
+                let expr = mkObjExpr(objtyR, baseValOpt, ctorCall, overrides', extraImpls, mWholeExpr)
+                mkCoerceIfNeeded g realObjTy objtyR expr
+            else
+                // Transform to avoid byref captures
+                TransformObjExprForStructByrefCaptures g mWholeExpr structCaptures objtyR baseValOpt ctorCall overrides' extraImpls realObjTy
+
         expr, tpenv
 
 //-------------------------------------------------------------------------
diff --git a/src/Compiler/Checking/Expressions/CheckExpressionsOps.fs b/src/Compiler/Checking/Expressions/CheckExpressionsOps.fs
@@ -2,6 +2,7 @@
 
 module internal FSharp.Compiler.CheckExpressionsOps
 
+open Internal.Utilities.Collections
 open Internal.Utilities.Library
 open Internal.Utilities.Library.Extras
 open FSharp.Compiler.CheckBasics
@@ -389,3 +390,128 @@ let inline mkOptionalParamTyBasedOnAttribute (g: TcGlobals.TcGlobals) tyarg attr
         mkValueOptionTy g tyarg
     else
         mkOptionTy g tyarg
+
+//-------------------------------------------------------------------------
+// Struct byref capture fix for object expressions
+//-------------------------------------------------------------------------
+
+/// When a struct instance method creates an object expression that captures constructor
+/// parameters, those captures go through 'this' which is a byref. This would create
+/// illegal byref fields in the closure class. This function detects such captures and
+/// extracts them to local bindings before the object expression.
+///
+/// Returns: (shouldTransform, structCaptures, methodParamStamps)
+let AnalyzeObjExprStructCaptures
+    (enclosingStructTyconRefOpt: TyconRef option)
+    (ctorCall: Expr)
+    (overrides: ObjExprMethod list)
+    (extraImpls: (TType * ObjExprMethod list) list)
+    : bool * Val list * Set<Stamp> =
+
+    // Collect free variables from an expression
+    let collectFreeVars expr =
+        (freeInExpr CollectLocals expr).FreeLocals |> Zset.elements
+
+    // Collect all method parameters (bound variables) from object expression methods
+    // These should NOT be treated as struct instance captures
+    let methodParams =
+        [
+            for TObjExprMethod(_, _, _, paramGroups, _, _) in overrides do
+                for paramGroup in paramGroups do
+                    for v in paramGroup do
+                        yield v
+            for (_, methods) in extraImpls do
+                for TObjExprMethod(_, _, _, paramGroups, _, _) in methods do
+                    for paramGroup in paramGroups do
+                        for v in paramGroup do
+                            yield v
+        ]
+        |> List.map (fun v -> v.Stamp)
+        |> Set.ofList
+
+    let allFreeVars =
+        [
+            yield! collectFreeVars ctorCall
+            for TObjExprMethod(_, _, _, _, body, _) in overrides do
+                yield! collectFreeVars body
+            for (_, methods) in extraImpls do
+                for TObjExprMethod(_, _, _, _, body, _) in methods do
+                    yield! collectFreeVars body
+        ]
+        |> List.distinctBy (fun v -> v.Stamp)
+
+    // Filter to struct instance captures:
+    // - We're in a struct context (enclosingStructTyconRefOpt is Some)
+    // - The value is NOT a method parameter of the object expression
+    // - The value is NOT a module binding
+    // - The value is NOT a member or module binding (excludes property getters, etc.)
+    // - The value is NOT a constructor
+    let structCaptures =
+        match enclosingStructTyconRefOpt with
+        | None -> []
+        | Some _ ->
+            allFreeVars
+            |> List.filter (fun v ->
+                not v.IsModuleBinding
+                && not v.IsMemberOrModuleBinding
+                && not (Set.contains v.Stamp methodParams)
+                && v.LogicalName <> ".ctor")
+
+    let shouldTransform = not (List.isEmpty structCaptures)
+    (shouldTransform, structCaptures, methodParams)
+
+/// Transform an object expression to avoid byref captures from struct instance state.
+/// Creates local bindings for captured values and remaps references in the object expression.
+let TransformObjExprForStructByrefCaptures
+    (g: TcGlobals.TcGlobals)
+    (mWholeExpr: Text.range)
+    (structCaptures: Val list)
+    (objtyR: TType)
+    (baseValOpt: Val option)
+    (ctorCall: Expr)
+    (overrides: ObjExprMethod list)
+    (extraImpls: (TType * ObjExprMethod list) list)
+    (realObjTy: TType)
+    : Expr =
+
+    // Create local bindings for each captured value to avoid byref captures
+    let localBindings =
+        structCaptures
+        |> List.map (fun v ->
+            let local, _localExpr =
+                mkCompGenLocal mWholeExpr (v.LogicalName + "$captured") v.Type
+
+            let readExpr = exprForVal mWholeExpr v
+            (v, local, readExpr))
+
+    // Build remap: original val -> local val
+    let remap =
+        localBindings
+        |> List.fold
+            (fun (r: Remap) (orig, local, _) ->
+                { r with
+                    valRemap = r.valRemap.Add orig (mkLocalValRef local)
+                })
+            Remap.Empty
+
+    // Helper to remap an object expression method
+    let remapMethod (TObjExprMethod(slotSig, attrs, mtps, paramGroups, body, range)) =
+        TObjExprMethod(slotSig, attrs, mtps, paramGroups, remapExpr g CloneAll remap body, range)
+
+    // Remap all parts of the object expression
+    let ctorCall' = remapExpr g CloneAll remap ctorCall
+    let overrides' = overrides |> List.map remapMethod
+
+    let extraImpls' =
+        extraImpls |> List.map (fun (ty, ms) -> (ty, ms |> List.map remapMethod))
+
+    // Build the object expression with remapped references
+    let objExpr =
+        mkObjExpr (objtyR, baseValOpt, ctorCall', overrides', extraImpls', mWholeExpr)
+
+    let objExpr = mkCoerceIfNeeded g realObjTy objtyR objExpr
+
+    // Wrap with let bindings: let x$captured = x in ...
+    localBindings
+    |> List.foldBack (fun (_, local, valueExpr) body -> mkLet DebugPointAtBinding.NoneAtInvisible mWholeExpr local valueExpr body)
+    <| objExpr
diff --git a/tests/FSharp.Compiler.ComponentTests/Conformance/Expressions/ObjectExpressions/StructObjectExpression.fs b/tests/FSharp.Compiler.ComponentTests/Conformance/Expressions/ObjectExpressions/StructObjectExpression.fs
@@ -0,0 +1,149 @@
+namespace FSharp.Compiler.ComponentTests.Conformance.Expressions
+
+open Xunit
+open FSharp.Test.Compiler
+
+module StructObjectExpression =
+
+    [<Fact>]
+    let ``Object expression in struct should not generate byref field - simple case`` () =
+        Fsx """
+type Class(test : obj) = class end
+
+[<Struct; NoComparison>]
+type Struct(test : obj) =
+    member _.Test() = {
+        new Class(test) with
+        member _.ToString() = ""
+    }
+
+let s = Struct(42)
+let obj = s.Test()
+        """
+        |> withOptions [ "--nowarn:52" ] // Suppress struct copy warning
+        |> compileExeAndRun
+        |> shouldSucceed
+
+    [<Fact>]
+    let ``Object expression in struct with multiple fields`` () =
+        Fsx """
+type Base(x: int, y: string) = class end
+
+[<Struct; NoComparison>]
+type MyStruct(x: int, y: string) =
+    member _.CreateObj() = {
+        new Base(x, y) with
+        member _.ToString() = y + string x
+    }
+
+let s = MyStruct(42, "test")
+let obj = s.CreateObj()
+        """
+        |> withOptions [ "--nowarn:52" ]
+        |> compileExeAndRun
+        |> shouldSucceed
+
+    [<Fact>]
+    let ``Object expression in struct referencing field in override method`` () =
+        Fsx """
+type IFoo =
+    abstract member DoSomething : unit -> int
+
+[<Struct; NoComparison>]
+type MyStruct(value: int) =
+    member _.CreateFoo() = {
+        new IFoo with
+        member _.DoSomething() = value * 2
+    }
+
+let s = MyStruct(21)
+let foo = s.CreateFoo()
+let result = foo.DoSomething()
+        """
+        |> withOptions [ "--nowarn:52" ]
+        |> compileExeAndRun
+        |> shouldSucceed
+
+    // Regression tests - these must continue to work
+
+    [<Fact>]
+    let ``Static member in struct with object expression should compile - StructBox regression`` () =
+        // This is the StructBox.Comparer pattern from FSharp.Core/seqcore.fs
+        // Static members don't have 'this' so should NOT be transformed
+        Fsx """
+open System.Collections.Generic
+
+[<Struct; NoComparison; NoEquality>]
+type StructBox<'T when 'T: equality>(value: 'T) =
+    member x.Value = value
+    
+    static member Comparer =
+        let gcomparer = HashIdentity.Structural<'T>
+        { new IEqualityComparer<StructBox<'T>> with
+            member _.GetHashCode(v) = gcomparer.GetHashCode(v.Value)
+            member _.Equals(v1, v2) = gcomparer.Equals(v1.Value, v2.Value) }
+
+let comparer = StructBox<int>.Comparer
+let box1 = StructBox(42)
+let box2 = StructBox(42)
+let result = comparer.Equals(box1, box2)
+if not result then failwith "Expected equal"
+        """
+        |> compileExeAndRun
+        |> shouldSucceed
+
+    [<Fact>]
+    let ``Module level object expression with struct parameter should compile`` () =
+        // Module-level functions don't have instance context
+        Fsx """
+[<Struct>]
+type MyStruct(value: int) =
+    member x.Value = value
+
+let createComparer () =
+    { new System.Object() with
+        member _.ToString() = "comparer" }
+
+let c = createComparer()
+if c.ToString() <> "comparer" then failwith "Failed"
+        """
+        |> compileExeAndRun
+        |> shouldSucceed
+
+    [<Fact>]
+    let ``Object expression in struct not capturing anything should compile`` () =
+        // Object expression that doesn't reference any struct state
+        Fsx """
+[<Struct; NoComparison>]
+type MyStruct(value: int) =
+    member _.CreateObj() = {
+        new System.Object() with
+        member _.ToString() = "constant"
+    }
+
+let s = MyStruct(42)
+let obj = s.CreateObj()
+if obj.ToString() <> "constant" then failwith "Failed"
+        """
+        |> withOptions [ "--nowarn:52" ]
+        |> compileExeAndRun
+        |> shouldSucceed
+
+    [<Fact>]
+    let ``Object expression in struct with method parameters should not confuse params with captures`` () =
+        // Method parameters are not instance captures, should not trigger transformation
+        Fsx """
+[<Struct; NoComparison>]
+type MyStruct(value: int) =
+    member _.Transform(multiplier: int) = {
+        new System.Object() with
+        member _.ToString() = string (value * multiplier)
+    }
+
+let s = MyStruct(21)
+let obj = s.Transform(2)
+if obj.ToString() <> "42" then failwith "Expected 42"
+        """
+        |> withOptions [ "--nowarn:52" ]
+        |> compileExeAndRun
+        |> shouldSucceed
diff --git a/tests/FSharp.Compiler.ComponentTests/FSharp.Compiler.ComponentTests.fsproj b/tests/FSharp.Compiler.ComponentTests/FSharp.Compiler.ComponentTests.fsproj
@@ -87,6 +87,7 @@
     <Compile Include="Conformance\Expressions\BindingExpressions\BindingExpressions.fs" />
     <Compile Include="Conformance\Expressions\ComputationExpressions\ComputationExpressions.fs" />
     <Compile Include="Conformance\Expressions\ObjectExpressions\ObjectExpressions.fs" />
+    <Compile Include="Conformance\Expressions\ObjectExpressions\StructObjectExpression.fs" />
     <Compile Include="Conformance\Expressions\ControlFlowExpressions\PatternMatching\PatternMatching.fs" />
     <Compile Include="Conformance\Expressions\ControlFlowExpressions\SequenceIteration\SequenceIteration.fs" />
     <Compile Include="Conformance\Expressions\ControlFlowExpressions\Type-relatedExpressions\Type-relatedExpressions.fs" />
