Bugfix :: Nullness :: Allow nullable return type for first branches of match and ifthenelse expressions

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

@@ -3,6 +3,7 @@
 * Fix Realsig+ generates nested closures with incorrect Generic  ([Issue #17797](https://github.com/dotnet/fsharp/issues/17797), [PR #17877](https://github.com/dotnet/fsharp/pull/17877))
 * Fix optimizer internal error for records with static fields  ([Issue #18165](https://github.com/dotnet/fsharp/issues/18165), [PR #18280](https://github.com/dotnet/fsharp/pull/18280))
 * Fix nullness warning with flexible types  ([Issue #18056](https://github.com/dotnet/fsharp/issues/18056), [PR #18266](https://github.com/dotnet/fsharp/pull/18266))
+* Allow first branches of match and if expressions to return nullable results  ([Issue #18015](https://github.com/dotnet/fsharp/issues/18015), [PR #18322](https://github.com/dotnet/fsharp/pull/18322))
 * Fix internal error when missing measure attribute in an unsolved measure typar. ([Issue #7491](https://github.com/dotnet/fsharp/issues/7491), [PR #18234](https://github.com/dotnet/fsharp/pull/18234)==
 * Set `Cancellable.token` from async computation ([Issue #18235](https://github.com/dotnet/fsharp/issues/18235), [PR #18238](https://github.com/dotnet/fsharp/pull/18238))
 * Fix missing nullness warning when static upcast dropped nullness ([Issue #18232](https://github.com/dotnet/fsharp/issues/18232), [PR #18261](https://github.com/dotnet/fsharp/pull/18261))

src/Compiler/Checking/Expressions/CheckExpressions.fs

@@ -10634,6 +10634,7 @@ and TcLinearExprs bodyChecker cenv env overallTy tpenv isCompExpr synExpr cont =
 
         | Some synElseExpr ->
             let env = { env with eContextInfo = ContextInfo.ElseBranchResult synElseExpr.Range }
+            TryAllowFlexibleNullnessInControlFlow (*isFirst=*) true g overallTy.Commit
             // tailcall
             TcLinearExprs bodyChecker cenv env overallTy tpenv isCompExpr synElseExpr (fun (elseExpr, tpenv) ->
                 let resExpr = primMkCond spIfToThen m overallTy.Commit boolExpr thenExpr elseExpr
@@ -10697,6 +10698,7 @@ and TcMatchClause cenv inputTy (resultTy: OverallTy) env isFirst tpenv synMatchC
         | DebugPointAtTarget.No -> resultEnv
 
     let resultExpr, tpenv = TcExprThatCanBeCtorBody cenv resultTy resultEnv tpenv synResultExpr
+    TryAllowFlexibleNullnessInControlFlow isFirst cenv.g resultTy.Commit
 
     let target = TTarget(vspecs, resultExpr, None)
 

src/Compiler/Checking/Expressions/CheckExpressionsOps.fs

@@ -18,6 +18,11 @@ open FSharp.Compiler.TypedTreeBasics
 open FSharp.Compiler.TypedTreeOps
 open FSharp.Compiler.SyntaxTreeOps
 
+let TryAllowFlexibleNullnessInControlFlow isFirst (g: TcGlobals.TcGlobals) ty =
+    match isFirst, g.checkNullness, GetTyparTyIfSupportsNull g ty with
+    | true, true, ValueSome tp -> tp.SetSupportsNullFlex(true)
+    | _ -> ()
+
 let CopyAndFixupTypars g m rigid tpsorig =
     FreshenAndFixupTypars g m rigid [] [] tpsorig
 

src/Compiler/TypedTree/TypedTreeOps.fs

@@ -9171,11 +9171,14 @@ let isReferenceTyparTy g ty =
     | ValueNone ->
         false
 
-let isSupportsNullTyparTy g ty = 
+let GetTyparTyIfSupportsNull g ty = 
     if isReferenceTyparTy g ty then
-        (destTyparTy g ty).Constraints |> List.exists (function TyparConstraint.SupportsNull _ -> true | _ -> false)
+        let tp = destTyparTy g ty
+        if tp.Constraints |> List.exists (function TyparConstraint.SupportsNull _ -> true | _ -> false) then
+            ValueSome tp
+        else ValueNone
     else
-        false
+        ValueNone
 
 let TypeNullNever g ty = 
     let underlyingTy = stripTyEqnsAndMeasureEqns g ty
@@ -9203,7 +9206,7 @@ let TypeNullIsExtraValue g m ty =
         | ValueNone -> 
 
         // Consider type parameters
-        isSupportsNullTyparTy g ty
+        (GetTyparTyIfSupportsNull g ty).IsSome
 
 // Any mention of a type with AllowNullLiteral(true) is considered to be with-null
 let intrinsicNullnessOfTyconRef g (tcref: TyconRef) =
@@ -9317,7 +9320,7 @@ let TypeNullIsExtraValueNew g m ty =
      | NullnessInfo.WithNull -> true)
     ||
     // Check if the type has a ': null' constraint
-    isSupportsNullTyparTy g ty
+    (GetTyparTyIfSupportsNull g ty).IsSome
 
 /// The pre-nullness logic about whether a type uses 'null' as a true representation value
 let TypeNullIsTrueValue g ty =

src/Compiler/TypedTree/TypedTreeOps.fsi

@@ -1829,6 +1829,8 @@ val TypeHasAllowNull: TyconRef -> TcGlobals -> range -> bool
 
 val TypeNullIsExtraValueNew: TcGlobals -> range -> TType -> bool
 
+val GetTyparTyIfSupportsNull: TcGlobals -> TType -> Typar voption
+
 val TypeNullNever: TcGlobals -> TType -> bool
 
 val TypeHasDefaultValue: TcGlobals -> range -> TType -> bool

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

@@ -146,6 +146,83 @@ let doNotWarnOnDowncastRepeatedNestedNullable(o:objnull) = o :? list<((AB | null
                   Error 3060, Line 7, Col 51, Line 7, Col 97, "This type test or downcast will erase the provided type 'List<AB | null array | null> | null' to the type 'List<AB array>'"]
 
 
+[<Fact>]
+let ``Can infer nullable type if first match handler returns null`` () =
+    FSharp """module TestLib
+
+let myFunc x =
+    match x with
+    | 0 -> null
+    | i -> "x"
+    """
+    |> asLibrary
+    |> typeCheckWithStrictNullness
+    |> shouldSucceed
+
+[<Fact>]
+let ``Can NOT infer nullable type if second match handler returns null`` () =
+    FSharp """module TestLib
+
+let myFunc x defaultValue =
+    match x with
+    | 0 -> defaultValue
+    | 1 -> null
+    | i -> "y"
+    """
+    |> asLibrary
+    |> typeCheckWithStrictNullness
+    |> shouldFail
+    |> withDiagnostics [Error 3261, Line 7, Col 12, Line 7, Col 15, "Nullness warning: The type 'string' does not support 'null'.. See also test.fs(7,11)-(7,14)."]
+
+[<Fact>]
+let ``Can infer nullable type if first match handler returns masked null`` () =
+    FSharp """module TestLib
+
+let thisIsNull : string|null = null
+let myFunc x =
+    match x with
+    | 0 -> thisIsNull
+    | i -> "x"
+    """
+    |> asLibrary
+    |> typeCheckWithStrictNullness
+    |> shouldSucceed
+
+[<Fact>]
+let ``Can infer nullable type from first branch of ifthenelse`` () =
+    FSharp """module TestLib
+
+let myFunc x =
+    if x = 0 then null else "x"
+    """
+    |> asLibrary
+    |> typeCheckWithStrictNullness
+    |> shouldSucceed
+
+[<Fact>]
+let ``Can NOT infer nullable type from second branch of ifthenelse`` () =
+    FSharp """module TestLib
+
+let myFunc x =
+    if x = 0 then "x" else null
+    """
+    |> asLibrary
+    |> typeCheckWithStrictNullness
+    |> shouldFail
+    |> withDiagnostics [Error 3261, Line 4, Col 28, Line 4, Col 32, "Nullness warning: The type 'string' does not support 'null'."]
+
+[<Fact>]
+let ``Can NOT infer nullable type from second branch of nested elifs`` () =
+    FSharp """module TestLib
+
+let myFunc x =
+    if x = 0 then "x" elif x=1 then null else "y"
+    """
+    |> asLibrary
+    |> typeCheckWithStrictNullness
+    |> shouldFail
+    |> withDiagnostics [Error 3261, Line 4, Col 37, Line 4, Col 41, "Nullness warning: The type 'string' does not support 'null'."]
+
 [<Fact>]
 let ``Can convert generic value to objnull arg`` () =
     FSharp """module TestLib