Feature nullness :: Try infer without null even when function/method arg is marked as nullable

src/Compiler/Checking/CheckExpressions.fs

@@ -483,8 +483,9 @@ let UnifyOverallType (cenv: cenv) (env: TcEnv) m overallTy actualTy =
     match overallTy with
     | MustConvertTo(isMethodArg, reqdTy) when g.langVersion.SupportsFeature LanguageFeature.AdditionalTypeDirectedConversions ->
         let actualTy = tryNormalizeMeasureInType g actualTy
-        let reqdTy = tryNormalizeMeasureInType g reqdTy
-        if AddCxTypeEqualsTypeUndoIfFailed env.DisplayEnv cenv.css m reqdTy actualTy then
+        let reqdTy = tryNormalizeMeasureInType g reqdTy       
+        let reqTyForUnification = reqTyForArgumentNullnessInference g actualTy reqdTy           
+        if AddCxTypeEqualsTypeUndoIfFailed env.DisplayEnv cenv.css m reqTyForUnification actualTy then
             ()
         else
             // try adhoc type-directed conversions

src/Compiler/Checking/ConstraintSolver.fs

@@ -1081,6 +1081,8 @@ and SolveNullnessSubsumesNullness (csenv: ConstraintSolverEnv) m2 (trace: Option
         SolveNullnessSubsumesNullness csenv m2 trace ty1 ty2 nv1.Solution nullness2
     | _, Nullness.Variable nv2 when nv2.IsSolved -> 
         SolveNullnessSubsumesNullness csenv m2 trace ty1 ty2 nullness1 nv2.Solution
+    | Nullness.Variable _nv1, Nullness.Known NullnessInfo.WithoutNull  -> 
+        CompleteD
     | Nullness.Variable nv1, _ -> 
         trace.Exec (fun () ->   nv1.Set nullness2) (fun () -> nv1.Unset())
         CompleteD
@@ -1414,6 +1416,8 @@ and SolveFunTypeEqn csenv ndeep m2 trace cxsln domainTy1 domainTy2 rangeTy1 rang
     trackErrors {
         // TODO NULLNESS: consider whether flipping the actual and expected in argument position
         // causes other problems, e.g. better/worse diagnostics
+        let g = csenv.g
+        let domainTy2 = reqTyForArgumentNullnessInference g domainTy1 domainTy2
         do! SolveTypeEqualsTypeKeepAbbrevsWithCxsln csenv ndeep m2 trace cxsln domainTy2 domainTy1
         return! SolveTypeEqualsTypeKeepAbbrevsWithCxsln csenv ndeep m2 trace cxsln rangeTy1 rangeTy2
     }

src/Compiler/Checking/MethodCalls.fs

@@ -480,7 +480,7 @@ let MakeCalledArgs amap m (minfo: MethInfo) minst =
         IsOutArg=isOutArg
         ReflArgInfo=reflArgInfo
         NameOpt=nmOpt
-        CalledArgumentType=calledArgTy })
+        CalledArgumentType= changeWithNullReqTyToVariable amap.g calledArgTy})
 
 /// <summary>
 /// Represents the syntactic matching between a caller of a method and the called method.

src/Compiler/TypedTree/TypedTreeBasics.fs

@@ -236,18 +236,22 @@ let rec stripUnitEqnsAux canShortcut unt =
     | _ -> unt
 
 let combineNullness (nullnessOrig: Nullness) (nullnessNew: Nullness) = 
-    match nullnessOrig.Evaluate() with
-    | NullnessInfo.WithoutNull -> nullnessNew
-    | NullnessInfo.AmbivalentToNull ->
-        match nullnessNew.Evaluate() with
-        | NullnessInfo.WithoutNull -> nullnessOrig
-        | NullnessInfo.AmbivalentToNull -> nullnessOrig
-        | NullnessInfo.WithNull -> nullnessNew
-    | NullnessInfo.WithNull -> 
-        match nullnessNew.Evaluate() with
-        | NullnessInfo.WithoutNull -> nullnessOrig
-        | NullnessInfo.AmbivalentToNull -> nullnessNew
-        | NullnessInfo.WithNull -> nullnessOrig
+    match nullnessOrig, nullnessNew with
+    | Nullness.Variable _, Nullness.Known NullnessInfo.WithoutNull -> 
+        nullnessOrig
+    | _ -> 
+        match nullnessOrig.Evaluate() with
+        | NullnessInfo.WithoutNull -> nullnessNew
+        | NullnessInfo.AmbivalentToNull ->
+            match nullnessNew.Evaluate() with
+            | NullnessInfo.WithoutNull -> nullnessOrig
+            | NullnessInfo.AmbivalentToNull -> nullnessOrig
+            | NullnessInfo.WithNull -> nullnessNew
+        | NullnessInfo.WithNull -> 
+            match nullnessNew.Evaluate() with
+            | NullnessInfo.WithoutNull -> nullnessOrig
+            | NullnessInfo.AmbivalentToNull -> nullnessNew
+            | NullnessInfo.WithNull -> nullnessOrig
 
 let nullnessEquiv (nullnessOrig: Nullness) (nullnessNew: Nullness) = LanguagePrimitives.PhysicalEquality nullnessOrig nullnessNew
 
@@ -278,8 +282,9 @@ let tryAddNullnessToTy nullnessNew (ty:TType) =
     | TType_measure _ -> None
 
 let addNullnessToTy (nullness: Nullness) (ty:TType) =
-    match nullness.Evaluate() with
-    | NullnessInfo.WithoutNull -> ty
+    match nullness with
+    | Nullness.Known NullnessInfo.WithoutNull -> ty
+    | Nullness.Variable nv when nv.IsSolved && nv.Evaluate() = NullnessInfo.WithoutNull -> ty
     | _ -> 
     match ty with
     | TType_var (tp, nullnessOrig) -> TType_var (tp, combineNullness nullnessOrig nullness)

src/Compiler/TypedTree/TypedTreeOps.fs

@@ -9157,11 +9157,33 @@ let nullnessOfTy g ty =
     |> function
         | TType_app(tcref, _, nullness) ->
             let nullness2 = intrinsicNullnessOfTyconRef g tcref
-            combineNullness nullness nullness2
+            if nullness2 === g.knownWithoutNull then
+                nullness
+            else
+                combineNullness nullness nullness2
         | TType_fun (_, _, nullness) | TType_var (_, nullness) ->
             nullness
         | _ -> g.knownWithoutNull
 
+let changeWithNullReqTyToVariable g reqTy =
+    let sty = stripTyEqns g reqTy
+    match isTyparTy g sty with
+    | false ->
+        match nullnessOfTy g sty with
+        | Nullness.Known NullnessInfo.WithNull -> 
+            reqTy |> replaceNullnessOfTy (NewNullnessVar())
+        | _ -> reqTy
+    | true -> reqTy
+
+/// When calling a null-allowing API, we prefer to infer a without null argument for idiomatic F# code.
+/// That is, unless caller explicitely marks a value (e.g. coming from a function parameter) as WithNull, it should not be infered as such.
+let reqTyForArgumentNullnessInference g actualTy reqTy =
+    // Only change reqd nullness if actualTy is an inference variable
+    match tryDestTyparTy g actualTy with
+    | ValueSome t when t.IsCompilerGenerated && not(t.Constraints |> List.exists(function | TyparConstraint.SupportsNull _ -> true | _ -> false))->
+        changeWithNullReqTyToVariable g reqTy       
+    | _ -> reqTy
+
 /// The new logic about whether a type admits the use of 'null' as a value.
 let TypeNullIsExtraValueNew g m ty = 
     let sty = stripTyparEqns ty

src/Compiler/TypedTree/TypedTreeOps.fsi

@@ -649,6 +649,10 @@ val tryDestForallTy: TcGlobals -> TType -> Typars * TType
 
 val nullnessOfTy: TcGlobals -> TType -> Nullness
 
+val changeWithNullReqTyToVariable: TcGlobals -> reqTy: TType -> TType
+
+val reqTyForArgumentNullnessInference: TcGlobals -> actualTy: TType -> reqTy: TType -> TType
+
 val isFunTy: TcGlobals -> TType -> bool
 
 val isForallTy: TcGlobals -> TType -> bool

tests/FSharp.Compiler.ComponentTests/EmittedIL/Nullness/ReferenceDU.fs

@@ -8,7 +8,7 @@ type MyDu =
 
 let giveMeLabel () = JustLabel
 
-let createMaybeString (innerValue) = MaybeString innerValue
+let createMaybeString (innerValue:string|null) = MaybeString innerValue
 
 let processNullableDu (x : (MyDu | null)) : string | null =
     match x with

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

@@ -29,6 +29,75 @@ let nonStrictFunc(x:string | null) = strictFunc(x)
     |> withDiagnostics [
         Error 3261, Line 4, Col 49, Line 4, Col 50, "Nullness warning: The types 'string' and 'string | null' do not have equivalent nullability."]
 
+[<Theory>]
+[<InlineData("fileExists(path)")>]
+[<InlineData("fileExists path")>]
+[<InlineData("fileExists null")>]
+[<InlineData("path |> fileExists")>]
+[<InlineData("null |> fileExists")>]
+[<InlineData("System.IO.File.Exists(path)")>]
+[<InlineData("System.IO.File.Exists(null)")>]
+[<InlineData("path |> System.IO.File.Exists")>]
+[<InlineData("null |> System.IO.File.Exists")>]
+[<InlineData("System.String.IsNullOrEmpty(path)")>]
+let ``Calling a nullAllowing API can still infer a withoutNull type``(functionCall) =
+    FSharp $"""
+module MyLib
+
+let myStrictFunc(x: string) = x.GetHashCode()
+let fileExists (path:string|null) = true
+
+let myStringReturningFunc (path) = 
+    let ex = {functionCall}
+    myStrictFunc(path)
+    """
+    |> asLibrary
+    |> typeCheckWithStrictNullness
+    |> shouldSucceed
+
+//[<Fact>]
+// TODO Tomas - as of now, this does not bring the desired result
+let ``Type inference with underscore or null`` () =
+    FSharp $"""
+module MyLib
+
+let myFunc (path: _ | null) =
+    System.IO.File.Exists(path)
+    """
+    |> asLibrary
+    |> typeCheckWithStrictNullness
+    |> shouldSucceed
+
+[<Fact>]
+let ``Type inference SystemIOFileExists`` () =
+    FSharp $"""
+module MyLib
+
+let test() = 
+    let maybeString : string | null = null
+    System.IO.File.Exists(maybeString)
+
+let myFunc path : string =
+    let exists =  path |> System.IO.File.Exists
+    path
+    """
+    |> asLibrary
+    |> typeCheckWithStrictNullness
+    |> shouldSucceed
+
+[<Fact>]
+let ``Type inference fsharp func`` () =
+    FSharp $"""module MyLib
+
+let fileExists (path:string|null) = true
+let myStringReturningFunc (pathArg) : string = 
+    let ex = pathArg |> fileExists
+    pathArg
+    """
+    |> asLibrary
+    |> typeCheckWithStrictNullness
+    |> shouldSucceed
+
 
 // P1: inline or not
 // P2: type annotation for function argument

tests/adhoc/nullness/enabled/positive.fs

@@ -92,10 +92,10 @@ module KonsoleWithNullsModule =
     let WriteLineC2(fmt: C | null, arg1: C | null) = Console.WriteLine(fmt.Value, arg1.Value)
 
 module KonsoleWithNullsModule2 = 
-    let WriteLine x = KonsoleWithNullsModule.WriteLine x
-    let WriteLine2 (fmt, arg1) = KonsoleWithNullsModule.WriteLine2(fmt, arg1)
-    let WriteLineC(s) = KonsoleWithNullsModule.WriteLineC(s)
-    let WriteLineC2(fmt, arg1) = KonsoleWithNullsModule.WriteLineC2(fmt, arg1)
+    let WriteLine (x : string | null) = KonsoleWithNullsModule.WriteLine x
+    let WriteLine2 (fmt: string | null, arg1: string | null) = KonsoleWithNullsModule.WriteLine2(fmt, arg1)
+    let WriteLineC(s: _ | null) = KonsoleWithNullsModule.WriteLineC(s)
+    let WriteLineC2(fmt: _ | null, arg1: _ | null) = KonsoleWithNullsModule.WriteLineC2(fmt, arg1)
 
 type KonsoleNoNulls = 
     static member WriteLine(s: String) = Console.WriteLine(s)