[RFC FS-1075] Improve interop to C# nullable-typed optionals

README.md

@@ -115,3 +115,4 @@ If you're curious about F# itself, check out these links:
 * [Get started with F#](https://docs.microsoft.com/dotnet/fsharp/get-started/)
 * [F# Software Foundation](https://fsharp.org)
 * [F# Testimonials](https://fsharp.org/testimonials)
+

src/fsharp/ConstraintSolver.fs

@@ -2209,14 +2209,15 @@ and ArgsMustSubsumeOrConvert
         trace
         cxsln
         isConstraint
+        enforceNullableOptionalsKnownTypes // use known types from nullable optional args?
         (calledArg: CalledArg) 
         (callerArg: CallerArg<'T>)  = trackErrors {
 
     let g = csenv.g
     let m = callerArg.Range
-    let calledArgTy = AdjustCalledArgType csenv.InfoReader isConstraint calledArg callerArg    
-    do! SolveTypeSubsumesTypeWithReport csenv ndeep m trace cxsln calledArgTy callerArg.Type
-    if calledArg.IsParamArray && isArray1DTy g calledArgTy && not (isArray1DTy g callerArg.Type) then 
+    let calledArgTy = AdjustCalledArgType csenv.InfoReader isConstraint enforceNullableOptionalsKnownTypes calledArg callerArg    
+    do! SolveTypeSubsumesTypeWithReport csenv ndeep m trace cxsln calledArgTy callerArg.CallerArgumentType
+    if calledArg.IsParamArray && isArray1DTy g calledArgTy && not (isArray1DTy g callerArg.CallerArgumentType) then 
         return! ErrorD(Error(FSComp.SR.csMethodExpectsParams(), m))
     else ()
   }
@@ -2228,14 +2229,14 @@ and MustUnifyInsideUndo csenv ndeep trace cxsln ty1 ty2 =
     SolveTypeEqualsTypeWithReport csenv ndeep csenv.m (WithTrace trace) cxsln ty1 ty2
 
 and ArgsMustSubsumeOrConvertInsideUndo (csenv: ConstraintSolverEnv) ndeep trace cxsln isConstraint calledArg (CallerArg(callerArgTy, m, _, _) as callerArg) = 
-    let calledArgTy = AdjustCalledArgType csenv.InfoReader isConstraint calledArg callerArg
+    let calledArgTy = AdjustCalledArgType csenv.InfoReader isConstraint true calledArg callerArg
     SolveTypeSubsumesTypeWithReport csenv ndeep  m (WithTrace trace) cxsln calledArgTy callerArgTy 
 
 and TypesMustSubsumeOrConvertInsideUndo (csenv: ConstraintSolverEnv) ndeep trace cxsln m calledArgTy callerArgTy = 
     SolveTypeSubsumesTypeWithReport csenv ndeep m trace cxsln calledArgTy callerArgTy 
 
 and ArgsEquivInsideUndo (csenv: ConstraintSolverEnv) isConstraint calledArg (CallerArg(callerArgTy, m, _, _) as callerArg) = 
-    let calledArgTy = AdjustCalledArgType csenv.InfoReader isConstraint calledArg callerArg
+    let calledArgTy = AdjustCalledArgType csenv.InfoReader isConstraint true calledArg callerArg
     if typeEquiv csenv.g calledArgTy callerArgTy then CompleteD else ErrorD(Error(FSComp.SR.csArgumentTypesDoNotMatch(), m))
 
 and ReportNoCandidatesError (csenv: ConstraintSolverEnv) (nUnnamedCallerArgs, nNamedCallerArgs) methodName ad (calledMethGroup: CalledMeth<_> list) isSequential =
@@ -2534,7 +2535,7 @@ and ResolveOverloading
                     // Prefer methods with more precise param array arg type
                     let c = 
                         if candidate.UsesParamArrayConversion && other.UsesParamArrayConversion then
-                            compareTypes candidate.ParamArrayElementType other.ParamArrayElementType
+                            compareTypes (candidate.GetParamArrayElementType()) (other.GetParamArrayElementType())
                         else
                             0
                     if c <> 0 then c else
@@ -2654,7 +2655,7 @@ and ResolveOverloading
                                  true
                                  (MustUnify csenv ndeep trace cxsln) 
                                  (TypesMustSubsumeOrConvertInsideUndo csenv ndeep trace cxsln m)// REVIEW: this should not be an "InsideUndo" operation
-                                 (ArgsMustSubsumeOrConvert csenv ndeep trace cxsln cx.IsSome) 
+                                 (ArgsMustSubsumeOrConvert csenv ndeep trace cxsln cx.IsSome true) 
                                  reqdRetTyOpt 
                                  calledMeth
                         | WithTrace calledMethTrc ->
@@ -2701,7 +2702,7 @@ let UnifyUniqueOverloading
             true // always check return type
             (MustUnify csenv ndeep NoTrace None) 
             (TypesMustSubsumeOrConvertInsideUndo csenv ndeep NoTrace None m)
-            (ArgsMustSubsumeOrConvert csenv ndeep NoTrace None false) // UnifyUniqueOverloading is not called in case of trait call - pass isConstraint=false 
+            (ArgsMustSubsumeOrConvert csenv ndeep NoTrace None false false) // UnifyUniqueOverloading is not called in case of trait call - pass isConstraint=false 
             (Some reqdRetTy)
             calledMeth
       return true

src/fsharp/FSharp.Core/option.fsi

@@ -173,7 +173,6 @@ module Option =
     [<CompiledName("ToList")>]
     val toList: option:'T option -> 'T list
 
-
     /// <summary>Convert the option to a Nullable value.</summary>
     /// <param name="option">The input option.</param>
     /// <returns>The result value.</returns>

src/fsharp/IlxGen.fs

@@ -5089,7 +5089,6 @@ and GenDecisionTreeSwitch cenv cgbuf inplabOpt stackAtTargets eenv e cases defau
             | _ -> error(InternalError("these matches should never be needed", switchm))
 
 and GenDecisionTreeCases cenv cgbuf stackAtTargets eenv defaultTargetOpt targets repeatSP targetInfos sequel caseLabels cases (contf: Zmap<_,_> -> FakeUnit) =
-    assert(cgbuf.GetCurrentStack() = stackAtTargets) // cgbuf stack should be unchanged over tests. [bug://1750].
 
     match defaultTargetOpt with
     | Some defaultTarget -> 

src/fsharp/LanguageFeatures.fs

@@ -28,6 +28,7 @@ type LanguageFeature =
     | PackageManagement
     | FromEndSlicing
     | FixedIndexSlice3d4d
+    | NullableOptionalInterop
 
 /// LanguageVersion management
 type LanguageVersion (specifiedVersionAsString) =
@@ -61,6 +62,7 @@ type LanguageVersion (specifiedVersionAsString) =
             LanguageFeature.OpenStaticClasses, previewVersion
             LanguageFeature.DotlessFloat32Literal, languageVersion50
             LanguageFeature.PackageManagement, previewVersion
+            LanguageFeature.NullableOptionalInterop, previewVersion
         ]
 
     let specified =

src/fsharp/LanguageFeatures.fsi

@@ -16,7 +16,7 @@ type LanguageFeature =
     | PackageManagement
     | FromEndSlicing
     | FixedIndexSlice3d4d
-
+    | NullableOptionalInterop
 
 /// LanguageVersion management
 type LanguageVersion =