Remove always-on NullableOptionalInterop language feature flag

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

@@ -200,6 +200,7 @@
 * Interpolated string holes (e.g. `$"{x}"`) are now formatted with invariant culture (via the `string` operator) instead of the current thread culture. ([PR #19971](https://github.com/dotnet/fsharp/pull/19971))
 * Lines starting with `#:` are now ignored ([Language suggestion 1440](https://github.com/fsharp/fslang-suggestions/issues/1440), [RFC FS-1337](https://github.com/fsharp/fslang-design/pull/830), [PR #20212](https://github.com/dotnet/fsharp/pull/20212))
 * Calculate Entity.PublicPath instead of storing ([PR #20285](https://github.com/dotnet/fsharp/pull/20285))
+* Remove the always-on `NullableOptionalInterop` language feature flag and collapse its guarded code to the enabled path; `--disableLanguageFeature:NullableOptionalInterop` is no longer a recognized feature name. ([Issue #20154](https://github.com/dotnet/fsharp/issues/20154), [PR #20378](https://github.com/dotnet/fsharp/pull/20378))
 
 ### Breaking Changes
 * Add `ExtendedLayoutAttribute` support for future .NET runtime interop. `ILTypeDefLayout` has a new `Extended` case. ([Issue #19190](https://github.com/dotnet/fsharp/issues/19190), [PR #19194](https://github.com/dotnet/fsharp/pull/19194))

src/Compiler/Checking/Expressions/CheckExpressions.fs

@@ -5883,7 +5883,7 @@ and TcAdjustExprForTypeDirectedConversions (cenv: cenv) (overallTy: OverallTy) a
     let g = cenv.g
 
     match overallTy with
-    | MustConvertTo (isMethodArg, reqdTy) when g.langVersion.SupportsFeature LanguageFeature.AdditionalTypeDirectedConversions || (g.langVersion.SupportsFeature LanguageFeature.NullableOptionalInterop && isMethodArg) ->
+    | MustConvertTo (isMethodArg, reqdTy) when g.langVersion.SupportsFeature LanguageFeature.AdditionalTypeDirectedConversions || isMethodArg ->
         let tcVal = LightweightTcValForUsingInBuildMethodCall g env.TraitContext
         AdjustExprForTypeDirectedConversions tcVal g cenv.amap cenv.infoReader env.AccessRights reqdTy actualTy m expr
     | _ ->

src/Compiler/Checking/MethodCalls.fs

@@ -296,7 +296,7 @@ let rec AdjustRequiredTypeForTypeDirectedConversions (infoReader: InfoReader) ad
     elif g.langVersion.SupportsFeature LanguageFeature.AdditionalTypeDirectedConversions && typeEquiv g g.float_ty reqdTy && typeEquiv g g.int32_ty actualTy then 
         g.int32_ty, TypeDirectedConversionUsed.Yes(warn TypeDirectedConversion.BuiltIn, false, false), None
 
-    elif g.langVersion.SupportsFeature LanguageFeature.NullableOptionalInterop && isMethodArg && isNullableTy g reqdTy && not (isNullableTy g actualTy) then 
+    elif isMethodArg && isNullableTy g reqdTy && not (isNullableTy g actualTy) then 
         let underlyingTy = destNullableTy g reqdTy
         // shortcut
         if typeEquiv g underlyingTy actualTy then
@@ -365,17 +365,13 @@ let AdjustCalledArgTypeForOptionals (infoReader: InfoReader) ad enforceNullableO
         match calledArg.OptArgInfo with
         // CSharpMethod(?x = arg), optional C#-style argument, may have nullable type
         | CallerSide _ ->
-            if g.langVersion.SupportsFeature LanguageFeature.NullableOptionalInterop then
-
-                let calledArgTy =
-                    if isNullableTy g calledArgTy then
-                        destNullableTy g calledArgTy
-                    else
-                        calledArgTy
+            let calledArgTy =
+                if isNullableTy g calledArgTy then
+                    destNullableTy g calledArgTy
+                else
+                    calledArgTy
 
-                mkOptionalTy g calledArgTy, TypeDirectedConversionUsed.No, None
-            else
-                calledArgTy, TypeDirectedConversionUsed.No, None
+            mkOptionalTy g calledArgTy, TypeDirectedConversionUsed.No, None
 
         // FSharpMethod(?x = arg), optional F#-style argument
         | CalleeSide ->
@@ -387,15 +383,11 @@ let AdjustCalledArgTypeForOptionals (infoReader: InfoReader) ad enforceNullableO
             AdjustCalledArgTypeForTypeDirectedConversionsAndAutoQuote infoReader ad callerArgTy calledArgTy calledArg m
     else
         match calledArg.OptArgInfo with 
-        // CSharpMethod(x = arg), non-optional C#-style argument, may have type Nullable<ty>. 
-        | NotOptional when not (g.langVersion.SupportsFeature LanguageFeature.NullableOptionalInterop) ->
-            AdjustCalledArgTypeForTypeDirectedConversionsAndAutoQuote infoReader ad callerArgTy calledArgTy calledArg m
-
         // The arg should have type ty. However for backwards compat, we also allow arg to have type Nullable<ty>
         | NotOptional 
         // CSharpMethod(x = arg), optional C#-style argument, may have type Nullable<ty>. 
         | CallerSide _ ->
-            if isNullableTy g calledArgTy && g.langVersion.SupportsFeature LanguageFeature.NullableOptionalInterop then 
+            if isNullableTy g calledArgTy then 
                 // If inference has worked out it's a nullable then use this
                 if isNullableTy g callerArgTy then
                     calledArgTy, TypeDirectedConversionUsed.No, None
@@ -1470,8 +1462,7 @@ let rec AdjustExprForTypeDirectedConversions tcVal (g: TcGlobals) amap infoReade
 
        mkCallToDoubleOperator g m actualTy expr
 
-   elif g.langVersion.SupportsFeature LanguageFeature.NullableOptionalInterop &&
-        isNullableTy g reqdTy && not (isNullableTy g actualTy) then
+   elif isNullableTy g reqdTy && not (isNullableTy g actualTy) then
 
        let underlyingTy = destNullableTy g reqdTy
        let adjustedExpr = AdjustExprForTypeDirectedConversions tcVal g amap infoReader ad underlyingTy actualTy m expr
@@ -1627,10 +1618,6 @@ let AdjustCallerArgForOptional tcVal tcFieldInit eCallerMemberName (infoReader:
     let reflArgInfo = calledArg.ReflArgInfo
     let calledArgTy = calledArg.CalledArgumentType
     match calledArg.OptArgInfo with
-    | NotOptional when not (g.langVersion.SupportsFeature LanguageFeature.NullableOptionalInterop) ->
-        if isOptCallerArg then errorR(Error(FSComp.SR.tcFormalArgumentIsNotOptional(), m))
-        assignedArg
-
     // For non-nullable, non-optional arguments no conversion is needed.
     // We return precisely the assignedArg.  This also covers the case where there
     // can be a lingering permitted type mismatch between caller argument and called argument, 

src/Compiler/Checking/OverloadResolutionRules.fs

@@ -319,13 +319,8 @@ let private compareArg (ctx: OverloadResolutionContext) (calledArg1: CalledArg)
                 // T is always better than inref<T>
                 | _ when isInByrefTy g ty2 && typeEquiv g ty1 (destByrefTy g ty2) -> true
 
-                // T is always better than Nullable<T> from F# 5.0 onwards
-                | _ when
-                    g.langVersion.SupportsFeature(LanguageFeature.NullableOptionalInterop)
-                    && isNullableTy g ty2
-                    && typeEquiv g ty1 (destNullableTy g ty2)
-                    ->
-                    true
+                // T is always better than Nullable<T>
+                | _ when isNullableTy g ty2 && typeEquiv g ty1 (destNullableTy g ty2) -> true
 
                 | _ -> false)
 
@@ -490,7 +485,7 @@ let private moreConcreteRule: TiebreakRule =
 let private nullableOptionalInteropRule: TiebreakRule =
     {
         Id = TiebreakRuleId.NullableOptionalInterop
-        RequiredFeature = Some LanguageFeature.NullableOptionalInterop
+        RequiredFeature = None
         Compare =
             fun ctx (struct (candidate, _, _)) (struct (other, _, _)) ->
                 let args1 = candidate.AllCalledArgs |> List.concat

src/Compiler/FSComp.txt

@@ -1568,7 +1568,6 @@ featurePackageManagement,"package management"
 featureFromEndSlicing,"from-end slicing"
 featureNullnessChecking,"nullness checking"
 featureResumableStateMachines,"resumable state machines"
-featureNullableOptionalInterop,"nullable optional interop"
 featureDefaultInterfaceMemberConsumption,"default interface member consumption"
 featureStringInterpolation,"string interpolation"
 featureWitnessPassing,"witness passing for trait constraints in F# quotations"

src/Compiler/Facilities/LanguageFeatures.fs

@@ -21,7 +21,6 @@ type LanguageFeature =
     | PackageManagement
     | FromEndSlicing
     | ResumableStateMachines
-    | NullableOptionalInterop
     | DefaultInterfaceMemberConsumption
     | WitnessPassing
     | AdditionalTypeDirectedConversions
@@ -139,7 +138,6 @@ type LanguageVersion(versionText, ?disabledFeaturesArray: LanguageFeature array)
             [
                 // F# 5.0
                 LanguageFeature.DotlessFloat32Literal, languageVersion50
-                LanguageFeature.NullableOptionalInterop, languageVersion50
                 LanguageFeature.DefaultInterfaceMemberConsumption, languageVersion50
                 LanguageFeature.PackageManagement, languageVersion50
                 LanguageFeature.WitnessPassing, languageVersion50
@@ -341,7 +339,6 @@ type LanguageVersion(versionText, ?disabledFeaturesArray: LanguageFeature array)
         | LanguageFeature.FromEndSlicing -> FSComp.SR.featureFromEndSlicing ()
         | LanguageFeature.NullnessChecking -> FSComp.SR.featureNullnessChecking ()
         | LanguageFeature.ResumableStateMachines -> FSComp.SR.featureResumableStateMachines ()
-        | LanguageFeature.NullableOptionalInterop -> FSComp.SR.featureNullableOptionalInterop ()
         | LanguageFeature.DefaultInterfaceMemberConsumption -> FSComp.SR.featureDefaultInterfaceMemberConsumption ()
         | LanguageFeature.WitnessPassing -> FSComp.SR.featureWitnessPassing ()
         | LanguageFeature.AdditionalTypeDirectedConversions -> FSComp.SR.featureAdditionalImplicitConversions ()

src/Compiler/Facilities/LanguageFeatures.fsi

@@ -11,7 +11,6 @@ type LanguageFeature =
     | PackageManagement
     | FromEndSlicing
     | ResumableStateMachines
-    | NullableOptionalInterop
     | DefaultInterfaceMemberConsumption
     | WitnessPassing
     | AdditionalTypeDirectedConversions