Bugfix :: Flexible types should subsume nullable version of equivalen…

…t CoarcesTo constraints (dotnet#18266)

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

@@ -1,6 +1,7 @@
 ### Fixed
 * 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))
 * 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/ConstraintSolver.fs

@@ -1609,7 +1609,10 @@ and SolveTyparSubtypeOfType (csenv: ConstraintSolverEnv) ndeep m2 trace tp ty1 =
     elif isSealedTy g ty1 then 
         SolveTypeEqualsTypeKeepAbbrevs csenv ndeep m2 trace (mkTyparTy tp) ty1
     else
-        AddConstraint csenv ndeep m2 trace tp (TyparConstraint.CoercesTo(ty1, csenv.m))
+        if SubtypeConstraintImplied g tp.Constraints ty1 then
+            CompleteD
+        else
+            AddConstraint csenv ndeep m2 trace tp (TyparConstraint.CoercesTo(ty1, csenv.m))
 
 and DepthCheck ndeep m = 
     if ndeep > 300 then 
@@ -2490,6 +2493,19 @@ and CheckConstraintImplication (csenv: ConstraintSolverEnv) tpc1 tpc2 =
 and CheckConstraintsImplication csenv existingConstraints newConstraint =
     existingConstraints |> List.exists (fun tpc2 -> CheckConstraintImplication csenv tpc2 newConstraint)
 
+and SubtypeConstraintImplied g existingConstraints newCoarceToTy =
+    if g.checkNullness then
+        let canBeNull t = (nullnessOfTy g t).Evaluate() = NullnessInfo.WithNull
+        let newTyIsWithoutNull = canBeNull newCoarceToTy |> not
+        let typeCoversNewConstraint existingTy = 
+            typeEquiv g existingTy newCoarceToTy
+            && not (newTyIsWithoutNull && canBeNull existingTy)   // :> T?  cannot imply :>T, since non-nullable is a stricter constraint.
+
+        existingConstraints 
+        |> List.exists (function | TyparConstraint.CoercesTo(ty2,_) when typeCoversNewConstraint ty2 -> true | _ -> false)
+    else
+        false
+
 // Ensure constraint conforms with existing constraints
 // NOTE: QUADRATIC
 and EnforceConstraintSetConsistency csenv ndeep m2 trace retry allCxs i cxs =

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

@@ -1003,7 +1003,45 @@ looseFunc(maybeTuple2) |> ignore
               Error 3261, Line 29, Col 12, Line 29, Col 19, "Nullness warning: The type 'MyDu | null' supports 'null' but a non-null type is expected."
               Error 3261, Line 30, Col 12, Line 30, Col 21, "Nullness warning: The type 'MyRecord | null' supports 'null' but a non-null type is expected."
               Error 43, Line 40, Col 36, Line 40, Col 40, "The type 'Maybe<int * int>' does not have 'null' as a proper value"]
-
+
+
+[<Fact>]
+let ``Nullness support for flexible types`` () =
+    FSharp """module MyLibrary
+open System
+let dispose (x: IDisposable | null) : unit =
+    match x with
+    | null -> ()
+    | d -> d.Dispose()
+
+let useThing (thing: #IDisposable) =
+    try
+        printfn "%O" thing
+    finally
+        dispose thing // Warning used to be here, should not warn! """
+    |> asLibrary
+    |> typeCheckWithStrictNullness
+    |> shouldSucceed
+
+[<Fact>]
+let ``Nullness support for flexible types - opposite`` () =
+    FSharp """module MyLibrary
+open System
+let dispose (x: IDisposable) : unit =
+    x.Dispose()
+
+let useThing (thing: #IDisposable | null) =
+    try
+        printfn "%O" thing
+    finally
+        dispose thing // Warning should be here, because 'thing' can be null!        
+        """
+    |> asLibrary
+    |> typeCheckWithStrictNullness
+    |> shouldFail
+    |> withDiagnostics [Error 3261, Line 10, Col 17, Line 10, Col 22, "Nullness warning: The types 'IDisposable' and ''a | null' do not have compatible nullability."]
+
+
 [<Fact>]
 let ``Static member on Record with null arg`` () =
     FSharp """module MyLibrary

tests/FSharp.Compiler.ComponentTests/Signatures/SeqTests.fs

@@ -2,6 +2,8 @@
 
 open Xunit
 open Signatures.TestHelpers
+open FSharp.Test
+open FSharp.Test.Compiler
 
 [<Fact>]
 let ``int seq`` () =
@@ -14,3 +16,17 @@ let ``tuple seq`` () =
     assertSingleSignatureBinding
         "let s = seq { yield (1, 'b', 2.) }"
         "val s: (int * char * float) seq"
+
+[<Fact>]
+let ``seq transpose`` () =
+    let encodeFs =
+        FsSource """module Program
+let transpose (source: seq<#seq<'T>>) =
+    source |> Seq.collect Seq.indexed |> Seq.groupBy fst |> Seq.map (snd >> (Seq.map snd))"""
+
+    Fsi """module Program
+val transpose: source: seq<'Collection> -> seq<seq<'T>> when 'Collection :> seq<'T>""" 
+    |> withAdditionalSourceFile encodeFs
+    |> withLangVersionPreview
+    |> compile
+    |> shouldSucceed