Optimizer: eliminate unused Unchecked.defaultof<'T> bindings (dotnet#19758)

* Fix EI_ilzero optimization: treat Unchecked.defaultof as effect-free for concrete types

Unchecked.defaultof<'T> (compiled as EI_ilzero) was unconditionally treated
as having an effect, preventing the optimizer from eliminating unused bindings
like `let _ = Unchecked.defaultof<decimal>`.

Fix: In ExprHasEffect and OptimizeExprOpFallback, EI_ilzero is now considered
effect-free when all type args are concrete (not type parameters). When type
variables are present (e.g. SRTP ^T), it is conservatively treated as having
an effect to prevent orphaned type variables during IL generation (FS0073).

Author: Copilot

docs/release-notes/.FSharp.Compiler.Service/11.0.100.md

@@ -8,6 +8,7 @@
 * Diagnostic FS0027 now emits a parameter-specific message (suggesting a `let mutable x = x` shadow or `byref<_>`) instead of the illegal `let mutable x = expression` shadow when the assignment target is a function or method parameter. ([Issue #15803](https://github.com/dotnet/fsharp/issues/15803), [PR #19866](https://github.com/dotnet/fsharp/pull/19866))
 * Recursive `inline` functions and members now emit a single clear error (FS3890) instead of a misleading FS1113/FS1114 optimizer cascade. ([Issue #17991](https://github.com/dotnet/fsharp/issues/17991), [PR #19803](https://github.com/dotnet/fsharp/pull/19803))
 * Report `FS0037 Duplicate definition of type or module` at type-check time when two sibling modules in a `module rec` / `namespace rec` group share a name, instead of letting the duplicate slip through to IL emit where it surfaced as the cryptic `FS2014: duplicate entry . in type index table`. ([Issue #6694](https://github.com/dotnet/fsharp/issues/6694), [PR #19913](https://github.com/dotnet/fsharp/pull/19913))
+* Unused `Unchecked.defaultof<'T>` bindings are now eliminated under optimization at their use sites. This removes redundant `initobj`/`ldnull;pop` sequences left behind after inlining SRTP helpers that use `Unchecked.defaultof` as dummy arguments to drive static-member-constraint resolution. Such bindings are preserved inside `inline` bodies (whose optimized form is pickled as cross-assembly optimization info and re-inlined by consumers) and whenever the erased type still references unsolved type variables, so no `FS0073` regression is introduced. ([Issue #18128](https://github.com/dotnet/fsharp/issues/18128), [PR #19758](https://github.com/dotnet/fsharp/pull/19758))
 * Semantic classification no longer marks recursive object self-references (`as this`, `let rec` self-refs) as mutable. ([Issue #5229](https://github.com/dotnet/fsharp/issues/5229))
 * Fix `MethodAccessException` under `--realsig+` when a closure (inner `let rec`, `task`/`async` state machine, or quotation splice) inside a member defined in an intrinsic type augmentation (`type C with member ...`) accesses a `private` member of `C`. The synthesized closure is now nested inside the declaring type instead of beside it in the module class. ([Issue #19933](https://github.com/dotnet/fsharp/issues/19933), [PR #19955](https://github.com/dotnet/fsharp/pull/19955))
 * Preserve source range for type errors on empty-bodied computation expressions (e.g. `foo {}`) in pipelines, function arguments, and type-annotated contexts, instead of reporting `unknown(1,1)`. ([Issue #19550](https://github.com/dotnet/fsharp/issues/19550), [PR #19849](https://github.com/dotnet/fsharp/pull/19849))

src/Compiler/CodeGen/IlxGen.fs

@@ -4708,7 +4708,7 @@ and GenApp (cenv: cenv) cgbuf eenv (f, fty, tyargs, curriedArgs, m) sequel =
                     (eenv, laterArgs)
                     ||> List.mapFold (fun eenv laterArg ->
                         // Only save arguments that have effects
-                        if Optimizer.ExprHasEffect g laterArg then
+                        if Optimizer.ExprHasEffect Optimizer.EffectContext.Emit g laterArg then
                             let ilTy = laterArg |> tyOfExpr g |> GenType cenv m eenv.tyenv
 
                             let locName =

src/Compiler/Optimize/Optimizer.fs

@@ -1617,36 +1617,68 @@ let SplitValuesByIsUsedOrHasEffect cenv fvs x =
 
 let IlAssemblyCodeInstrHasEffect i = 
     match i with 
-    | ( AI_nop | AI_ldc _ | AI_add | AI_sub | AI_mul | AI_xor | AI_and | AI_or 
-               | AI_ceq | AI_cgt | AI_cgt_un | AI_clt | AI_clt_un | AI_conv _ | AI_shl 
-               | AI_shr | AI_shr_un | AI_neg | AI_not | AI_ldnull )
+    | AI_nop | AI_ldc _ | AI_add | AI_sub | AI_mul | AI_xor | AI_and | AI_or
+    | AI_ceq | AI_cgt | AI_cgt_un | AI_clt | AI_clt_un | AI_conv _ | AI_shl
+    | AI_shr | AI_shr_un | AI_neg | AI_not | AI_ldnull
     | I_ldstr _ | I_ldtoken _ -> false
     | _ -> true
-  
+
 let IlAssemblyCodeHasEffect instrs = List.exists IlAssemblyCodeInstrHasEffect instrs
 
-let rec ExprHasEffect g expr = 
+/// The context an expression's effects are analyzed in. It governs whether a fully-ground
+/// `Unchecked.defaultof` (an `EI_ilzero` witness) may be treated as effect-free and its unused
+/// binding dropped: safe when emitting here, but not inside an `inline` value's body, whose
+/// optimized form is pickled as cross-assembly info and re-inlined at each use site — dropping
+/// the witness there corrupts that info and trips FS0073 in the consumer's IlxGen (e.g. the SRTP
+/// witness/dummy arguments used pervasively by FSharpPlus).
+[<RequireQualifiedAccess>]
+type EffectContext =
+    /// Emitting IL here, at a fully instantiated use site.
+    | Emit
+    /// Analyzing the pickled body of an `inline` value, re-inlined at other sites.
+    | InlineBody
+
+let ILAsmWithIlzeroHasEffect context instrs tyargs =
+    let hasIlzero = instrs |> List.exists (function EI_ilzero _ -> true | _ -> false)
+
+    if not hasIlzero then
+        IlAssemblyCodeHasEffect instrs
+    else
+        // A fully-ground ilzero is a pure default-value load whose unused binding may be dropped,
+        // but only at emission and only when no tyarg still holds a free typar: such a binding is
+        // the sole pin for those typars and dropping it leaves them unsolved (FS0073).
+        let ilzeroIsSafeToDrop =
+            match context with
+            | EffectContext.Emit -> tyargs |> List.forall (fun ty -> Zset.isEmpty (freeInType CollectTypars ty).FreeTypars)
+            | EffectContext.InlineBody -> false
+
+        let otherInstrsHaveEffect =
+            instrs |> List.exists (function EI_ilzero _ -> false | i -> IlAssemblyCodeInstrHasEffect i)
+
+        otherInstrsHaveEffect || not ilzeroIsSafeToDrop
+
+let rec ExprHasEffect context g expr = 
     match stripDebugPoints expr with 
     | Expr.Val (vref, _, _) -> vref.IsTypeFunction || vref.IsMutable
     | Expr.Quote _ 
     | Expr.Lambda _
     | Expr.TyLambda _ 
     | Expr.Const _ -> false
     // type applications do not have effects, with the exception of type functions
-    | Expr.App (f0, _, _, [], _) -> IsTyFuncValRefExpr f0 || ExprHasEffect g f0
-    | Expr.Op (op, _, args, m) -> ExprsHaveEffect g args || OpHasEffect g m op
-    | Expr.LetRec (binds, body, _, _) -> BindingsHaveEffect g binds || ExprHasEffect g body
-    | Expr.Let (bind, body, _, _) -> BindingHasEffect g bind || ExprHasEffect g body
+    | Expr.App (f0, _, _, [], _) -> IsTyFuncValRefExpr f0 || ExprHasEffect context g f0
+    | Expr.Op (op, tyargs, args, m) -> ExprsHaveEffect context g args || OpHasEffect context g m op tyargs
+    | Expr.LetRec (binds, body, _, _) -> BindingsHaveEffect context g binds || ExprHasEffect context g body
+    | Expr.Let (bind, body, _, _) -> BindingHasEffect context g bind || ExprHasEffect context g body
     // REVIEW: could add Expr.Obj on an interface type - these are similar to records of lambda expressions 
     | _ -> true
 
-and ExprsHaveEffect g exprs = List.exists (ExprHasEffect g) exprs
+and ExprsHaveEffect context g exprs = List.exists (ExprHasEffect context g) exprs
 
-and BindingsHaveEffect g binds = List.exists (BindingHasEffect g) binds
+and BindingsHaveEffect context g binds = List.exists (BindingHasEffect context g) binds
 
-and BindingHasEffect g bind = bind.Expr |> ExprHasEffect g
+and BindingHasEffect context g bind = bind.Expr |> ExprHasEffect context g
 
-and OpHasEffect g m op = 
+and OpHasEffect context g m op tyargs =
     match op with 
     | TOp.Tuple _ -> false
     | TOp.AnonRecd _ -> false
@@ -1660,7 +1692,7 @@ and OpHasEffect g m op =
     | TOp.UnionCaseTagGet _ -> false
     | TOp.UnionCaseProof _ -> false
     | TOp.UnionCaseFieldGet (ucref, n) -> isUnionCaseFieldMutable g ucref n 
-    | TOp.ILAsm (instrs, _) -> IlAssemblyCodeHasEffect instrs
+    | TOp.ILAsm (instrs, _) -> ILAsmWithIlzeroHasEffect context instrs tyargs
     | TOp.TupleFieldGet _ -> false
     | TOp.ExnFieldGet (ecref, n) -> isExnFieldMutable ecref n 
     | TOp.RefAddrGet _ -> false
@@ -1687,6 +1719,9 @@ and OpHasEffect g m op =
     | TOp.LValueOp _ (* conservative *)
     | TOp.ValFieldSet _ -> true
 
+let effectContextOf (cenv: cenv) =
+    if cenv.optimizing then EffectContext.Emit else EffectContext.InlineBody
+
 
 let TryEliminateBinding cenv _env bind e2 _m =
     let g = cenv.g
@@ -1717,7 +1752,7 @@ let TryEliminateBinding cenv _env bind e2 _m =
               match argsr with 
               | Expr.Val (VRefLocal vspec2, _, _) :: argsr2
                  when valEq vspec1 vspec2 && IsUniqueUse vspec2 (List.rev rargsl@argsr2) -> Some(List.rev rargsl, argsr2)
-              | argsrh :: argsrt when not (ExprHasEffect g argsrh) -> GetImmediateUseContext (argsrh :: rargsl) argsrt 
+              | argsrh :: argsrt when not (ExprHasEffect (effectContextOf cenv) g argsrh) -> GetImmediateUseContext (argsrh :: rargsl) argsrt 
               | _ -> None
 
         let (DebugPoints(e2, recreate0)) = e2
@@ -2603,7 +2638,7 @@ and OptimizeExprOp cenv env (op, tyargs, args, m) =
         newExpr,
         { TotalSize = 1
           FunctionSize = 1
-          HasEffect = OpHasEffect g m newOp
+          HasEffect = OpHasEffect (effectContextOf cenv) g m newOp tyargs
           MightMakeCriticalTailcall = false
           Info = ValueOfExpr newExpr }
 
@@ -2680,7 +2715,7 @@ and OptimizeExprOpFallback cenv env (op, tyargs, argsR, m) arginfos value_ =
     let argsFSize = AddFunctionSizes arginfos
     let argEffects = OrEffects arginfos
     let argValues = List.map (fun x -> x.Info) arginfos
-    let effect = OpHasEffect g m op
+    let effect = OpHasEffect (effectContextOf cenv) g m op tyargs
     let cost, value_ = 
       match op with
       | TOp.UnionCase c -> 2, MakeValueInfoForUnionCase c (Array.ofList argValues)

src/Compiler/Optimize/Optimizer.fsi

@@ -110,8 +110,16 @@ val AbstractOptimizationInfoToEssentials: (CcuOptimizationInfo -> CcuOptimizatio
 /// Combine optimization infos
 val UnionOptimizationInfos: seq<ImplFileOptimizationInfo> -> CcuOptimizationInfo
 
+/// The context an expression's effects are analyzed in: emitting IL here, or analyzing the
+/// pickled body of an `inline` value. Governs whether a fully-ground `Unchecked.defaultof`
+/// binding may be eliminated.
+[<RequireQualifiedAccess>]
+type EffectContext =
+    | Emit
+    | InlineBody
+
 /// Check if an expression has an effect
-val ExprHasEffect: TcGlobals -> Expr -> bool
+val ExprHasEffect: EffectContext -> TcGlobals -> Expr -> bool
 
 val internal u_CcuOptimizationInfo: ReaderState -> CcuOptimizationInfo
 

tests/FSharp.Compiler.ComponentTests/EmittedIL/UncheckedDefaultofOptimization.fs

@@ -0,0 +1,126 @@
+// Copyright (c) Microsoft Corporation.  All Rights Reserved.  See License.txt in the project root for license information.
+
+namespace EmittedIL
+
+open Xunit
+open FSharp.Test.Compiler
+
+module ``UncheckedDefaultofOptimization`` =
+
+    // https://github.com/dotnet/fsharp/issues/18128
+    // Unused `Unchecked.defaultof<concreteType>` bindings should be eliminated under optimization.
+    // Pins both the absence of `initobj` and that no decimal local slot is allocated for any of the
+    // three discarded bindings.
+    [<Fact>]
+    let ``Unused Unchecked.defaultof bindings of concrete types are eliminated`` () =
+        FSharp """
+module Test
+open System
+let f (n: float32) =
+    Console.WriteLine n
+    let _ = Unchecked.defaultof<decimal>
+    let _ = Unchecked.defaultof<decimal>
+    let _ = Unchecked.defaultof<decimal>
+    let n' = n * 2.f
+    Console.WriteLine n'
+        """
+        |> withOptimize
+        |> asLibrary
+        |> compile
+        |> shouldSucceed
+        |> verifyILNotPresent [ "initobj"; "valuetype [runtime]System.Decimal" ]
+
+    // https://github.com/dotnet/fsharp/issues/18128
+    // The FSharpPlus-style SRTP witness pattern from the issue. Elimination happens at the (fully
+    // instantiated) use site: `doWork` reduces to a direct multiplication with the `nil<PreOps>` and
+    // `nil< ^b >` witness bindings gone. The assertion is scoped to `doWork` rather than the whole
+    // module because the inline functions also emit a compiler-generated dynamic-invocation stub which
+    // legitimately retains an `ldnull` (see the guard below and issue #19758 - the witness bindings of
+    // an *inline* body must be preserved so cross-assembly consumers can re-inline and specialize them).
+    [<Fact>]
+    let ``Unused Unchecked.defaultof SRTP witness bindings are eliminated at the use site`` () =
+        FSharp """
+module Test
+
+open System.ComponentModel
+
+[<AbstractClass; Sealed; EditorBrowsable(EditorBrowsableState.Never)>]
+type PreOps =
+    static member inline Double (n: float<'u>) : float<'u> = n * 2.
+    static member inline Double (n: float32<'u>) : float32<'u> = n * 2.f
+
+#nowarn "64"
+module PreludeOperators =
+    let inline private nil<'T> = Unchecked.defaultof<'T>
+    let inline double (x: ^a) =
+        let inline _call (_: ^M, input: ^I, _: ^R) = ((^M or ^I) : (static member Double : ^I -> ^R) input)
+        _call (nil<PreOps>, x, nil< ^b >)
+
+open PreludeOperators
+let doWork (n: float) = double n
+        """
+        |> withOptimize
+        |> asLibrary
+        |> compile
+        |> shouldSucceed
+        |> verifyIL [
+            """.method public static float64  doWork(float64 n) cil managed
+  {
+    
+    .maxstack  8
+    IL_0000:  ldarg.0
+    IL_0001:  ldc.r8     2.
+    IL_000a:  mul
+    IL_000b:  ret
+  }"""
+        ]
+
+    // https://github.com/dotnet/fsharp/issues/18128
+    // Soundness pin: eliminating an unused `Unchecked.defaultof<T>` must not introduce a new reference
+    // to T in the enclosing method. `defaultof` of a reference type lowers to `ldnull` (not `newobj`),
+    // so removing the binding cannot suppress an observable cctor call - `f`'s body must contain no
+    // reference to `WithCctor` at all.
+    [<Fact>]
+    let ``Eliminated Unchecked.defaultof leaves no reference to T in the caller`` () =
+        FSharp """
+module Test
+
+type WithCctor() =
+    static do failwith "cctor must not run"
+
+let f () =
+    let _ = Unchecked.defaultof<WithCctor>
+    42
+        """
+        |> withOptimize
+        |> asLibrary
+        |> compile
+        |> shouldSucceed
+        |> verifyIL [
+            """.method public static int32  f() cil managed
+  {
+    
+    .maxstack  8
+    IL_0000:  ldc.i4.s   42
+    IL_0002:  ret
+  }"""
+        ]
+
+    // https://github.com/dotnet/fsharp/issues/18128
+    // Regression for SRTP witness/dummy-argument patterns (e.g. FSharpPlus) where eliminating an
+    // `Unchecked.defaultof` binding whose type still references unsolved typars would trip FS0073 in
+    // IlxGen. Such bindings must be kept; only fully-ground ilzero bindings are removed.
+    [<Fact>]
+    let ``Unused Unchecked.defaultof bindings of unsolved generic types do not cause FS0073`` () =
+        FSharp """
+module Test
+let inline witness () = Unchecked.defaultof<'T>
+let inline run< ^T when ^T: (static member Zero: ^T)> () =
+    let _ = (witness () : ^T)
+    LanguagePrimitives.GenericZero< ^T>
+let v : int = run< int> ()
+        """
+        |> withOptimize
+        |> asLibrary
+        |> compile
+        |> shouldSucceed

tests/FSharp.Compiler.ComponentTests/FSharp.Compiler.ComponentTests.fsproj

@@ -252,6 +252,7 @@
     <!--<Compile Include="EmittedIL\StructGettersReadOnly.fs" />-->
     <Compile Include="EmittedIL\TailCalls.fs" />
     <Compile Include="EmittedIL\TupleElimination.fs" />
+    <Compile Include="EmittedIL\UncheckedDefaultofOptimization.fs" />
     <Compile Include="EmittedIL\TypeTestsInPatternMatching.fs" />
     <Compile Include="EmittedIL\WhileLoops.fs" />
     <Compile Include="EmittedIL\ArgumentNames.fs" />