Bugfix :: Support for 'use' on a nullable IDisposable (#18262)

* Support for 'use' on a nullable IDisposable

* release notes

Author: T-Gro

docs/release-notes/.FSharp.Compiler.Service/9.0.300.md

@@ -7,6 +7,7 @@
 
 ### Added
 * Added missing type constraints in FCS. ([PR #18241](https://github.com/dotnet/fsharp/pull/18241))
+* The 'use' keyword can be used on IDisposable|null without nullness warnings ([PR #18262](https://github.com/dotnet/fsharp/pull/18262))
 
 ### Changed
 

src/Compiler/Checking/Expressions/CheckExpressions.fs

@@ -3097,7 +3097,7 @@ let BuildDisposableCleanup (cenv: cenv) env m (v: Val) =
         else
             mkUnit g m
     else
-        let disposeObjVar, disposeObjExpr = mkCompGenLocal m "objectToDispose" g.system_IDisposable_ty
+        let disposeObjVar, disposeObjExpr = mkCompGenLocal m "objectToDispose" g.system_IDisposableNull_ty
         let disposeExpr, _ = BuildPossiblyConditionalMethodCall cenv env PossiblyMutates m false disposeMethod NormalValUse [] [disposeObjExpr] [] None
         let inputExpr = mkCoerceExpr(exprForVal v.Range v, g.obj_ty_ambivalent, m, v.Type)
         mkIsInstConditional g m g.system_IDisposable_ty inputExpr disposeObjVar disposeExpr (mkUnit g m)
@@ -6811,7 +6811,7 @@ and TcCtorCall isNaked cenv env tpenv (overallTy: OverallTy) objTy mObjTyOpt ite
     match item, args with
     | Item.CtorGroup(methodName, minfos), _ ->
         let meths = List.map (fun minfo -> minfo, None) minfos
-        if isNaked && TypeFeasiblySubsumesType 0 g cenv.amap mWholeCall g.system_IDisposable_ty NoCoerce objTy then
+        if isNaked && TypeFeasiblySubsumesType 0 g cenv.amap mWholeCall g.system_IDisposableNull_ty NoCoerce objTy then
             warning(Error(FSComp.SR.tcIDisposableTypeShouldUseNew(), mWholeCall))
 
         // Check the type is not abstract
@@ -11603,7 +11603,7 @@ and TcLetBinding (cenv: cenv) isUse env containerInfo declKind tpenv (synBinds,
                 let isDiscarded = match checkedPat2 with TPat_wild _ -> true | _ -> false
                 let allValsDefinedByPattern = if isDiscarded then [patternInputTmp] else allValsDefinedByPattern
                 (allValsDefinedByPattern, (bodyExpr, bodyExprTy)) ||> List.foldBack (fun v (bodyExpr, bodyExprTy) ->
-                    AddCxTypeMustSubsumeType ContextInfo.NoContext denv cenv.css v.Range NoTrace g.system_IDisposable_ty v.Type
+                    AddCxTypeMustSubsumeType ContextInfo.NoContext denv cenv.css v.Range NoTrace g.system_IDisposableNull_ty v.Type
                     let cleanupE = BuildDisposableCleanup cenv env m v
                     mkTryFinally g (bodyExpr, cleanupE, m, bodyExprTy, DebugPointAtTry.No, DebugPointAtFinally.No), bodyExprTy)
             else

src/Compiler/Checking/Expressions/CheckExpressionsOps.fs

@@ -297,7 +297,7 @@ let mkSeqEmpty (cenv: TcFileState) env m genTy =
 
 let mkSeqUsing (cenv: TcFileState) (env: TcEnv) m resourceTy genTy resourceExpr lam =
     let g = cenv.g
-    AddCxTypeMustSubsumeType ContextInfo.NoContext env.DisplayEnv cenv.css m NoTrace g.system_IDisposable_ty resourceTy
+    AddCxTypeMustSubsumeType ContextInfo.NoContext env.DisplayEnv cenv.css m NoTrace g.system_IDisposableNull_ty resourceTy
     let genResultTy = NewInferenceType g
     UnifyTypes cenv env m genTy (mkSeqTy cenv.g genResultTy)
     mkCallSeqUsing cenv.g m resourceTy genResultTy resourceExpr lam

src/Compiler/Optimize/LowerComputedCollections.fs

@@ -35,7 +35,7 @@ let BuildDisposableCleanup tcVal (g: TcGlobals) infoReader m (v: Val) =
 
         disposeExpr
     else
-        let disposeObjVar, disposeObjExpr = mkCompGenLocal m "objectToDispose" g.system_IDisposable_ty
+        let disposeObjVar, disposeObjExpr = mkCompGenLocal m "objectToDispose" g.system_IDisposableNull_ty
         let disposeExpr, _ = BuildMethodCall tcVal g infoReader.amap PossiblyMutates m false disposeMethod NormalValUse [] [disposeObjExpr] [] None
         let inputExpr = mkCoerceExpr(exprForVal v.Range v, g.obj_ty_ambivalent, m, v.Type)
         mkIsInstConditional g m g.system_IDisposable_ty inputExpr disposeObjVar disposeExpr (mkUnit g m)

src/Compiler/TypedTree/TcGlobals.fs

@@ -1369,6 +1369,7 @@ type TcGlobals(
   member val system_Array_ty = mkSysNonGenericTy sys "Array"
   member val system_Object_ty = mkSysNonGenericTy sys "Object"
   member val system_IDisposable_ty = mkSysNonGenericTy sys "IDisposable"
+  member val system_IDisposableNull_ty = mkNonGenericTyWithNullness (findSysTyconRef sys "IDisposable") v_knownWithNull
   member val system_RuntimeHelpers_ty = mkSysNonGenericTy sysCompilerServices "RuntimeHelpers"
   member val system_Value_ty = mkSysNonGenericTy sys "ValueType"
   member val system_Delegate_ty = mkSysNonGenericTy sys "Delegate"

src/Compiler/TypedTree/TcGlobals.fsi

@@ -1199,6 +1199,8 @@ type internal TcGlobals =
 
     member system_IDisposable_ty: FSharp.Compiler.TypedTree.TType
 
+    member system_IDisposableNull_ty: FSharp.Compiler.TypedTree.TType
+
     member system_IFormattable_tcref: FSharp.Compiler.TypedTree.EntityRef
 
     member system_IFormattable_ty: FSharp.Compiler.TypedTree.TType

tests/FSharp.Compiler.ComponentTests/Language/Nullness/NullableReferenceTypesTests.fs

@@ -173,6 +173,19 @@ let safeHolder : IDisposable =
     |> typeCheckWithStrictNullness
     |> shouldSucceed
 
+[<Fact>]
+let ``Can _use_ a nullable IDisposable`` () =
+    FSharp """module TestLib
+open System
+let workWithResource (getD:int -> (IDisposable|null)) =
+    use _ = getD 15
+    15
+
+    """
+    |> asLibrary
+    |> typeCheckWithStrictNullness
+    |> shouldSucceed
+
 [<Fact>]
 let ``Does not duplicate warnings`` () =
     FSharp """