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Bugfix: Warn when upcast drops nullness via FindUniqueFeasibleSupertype #18261

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Bugfix: Warn when upcast drops nullness via FindUniqueFeasibleSupertype #18261

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12 changes: 10 additions & 2 deletions src/Compiler/Checking/TypeRelations.fs
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Original file line number Diff line number Diff line change
Expand Up @@ -341,5 +341,13 @@ let IteratedAdjustLambdaToMatchValReprInfo g amap valReprInfo lambdaExpr =
/// "Single Feasible Type" inference
/// Look for the unique supertype of ty2 for which ty2 :> ty1 might feasibly hold
let FindUniqueFeasibleSupertype g amap m ty1 ty2 =
let supertypes = Option.toList (GetSuperTypeOfType g amap m ty2) @ (GetImmediateInterfacesOfType SkipUnrefInterfaces.Yes g amap m ty2)
supertypes |> List.tryFind (TypeFeasiblySubsumesType 0 g amap m ty1 NoCoerce)
let n2 = nullnessOfTy g ty2
let nullify t = addNullnessToTy n2 t

let supertypes =
Option.toList (GetSuperTypeOfType g amap m ty2) @
(GetImmediateInterfacesOfType SkipUnrefInterfaces.Yes g amap m ty2)

supertypes
|> List.tryFind (TypeFeasiblySubsumesType 0 g amap m ty1 NoCoerce)
|> Option.map nullify
14 changes: 14 additions & 0 deletions tests/FSharp.Compiler.ComponentTests/Language/Nullness/NullableReferenceTypesTests.fs
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Expand Up @@ -17,6 +17,20 @@ let typeCheckWithStrictNullness cu =
|> withNullnessOptions
|> typecheck

[<Fact>]
let ``Warning on nullness hidden behind interface upcast`` () =
FSharp """module Test

open System.IO
open System

// This is bad - input is nullable, output is not = must warn
let whatisThis (s:Stream|null) : IDisposable =
s"""
|> asLibrary
|> typeCheckWithStrictNullness
|> shouldFail
|> withDiagnostics [Error 3261, Line 8, Col 5, Line 8, Col 6, "Nullness warning: The types 'IDisposable' and 'IDisposable | null' do not have compatible nullability."]

[<FSharp.Test.FactForNETCOREAPPAttribute>]
let ``Report warning when applying anon record to a nullable generic return value`` () =
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