Skip to content

Fix IEquatable<T> nullness contravariance to match C# behavior #18770

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Merged
merged 10 commits into from
Jul 25, 2025
Merged

Fix IEquatable<T> nullness contravariance to match C# behavior #18770

Show file tree
Hide file tree
Changes from 8 commits
Commits
Show all changes
10 commits
Select commit Hold shift + click to select a range
38e2a17
Initial plan
Copilot Jul 21, 2025
281f229
Implement IEquatable<T> nullness contravariance fix
Copilot Jul 21, 2025
f97affd
Final validation of IEquatable<T> nullness contravariance fix
Copilot Jul 21, 2025
89b26ab
Update tests/FSharp.Compiler.ComponentTests/Language/Nullness/Nullabl…
T-Gro Jul 22, 2025
ad2a833
Simplify IEquatable contravariance logic per code review feedback
Copilot Jul 22, 2025
5c8ea05
Remove StandardOutput.txt and StandardError.txt files and add them to…
Copilot Jul 22, 2025
58dddd6
Merge branch 'main' into copilot/fix-20e8251c-e9bc-4713-977e-9b992f4c…
T-Gro Jul 22, 2025
2d07f3d
Add release notes entry for IEquatable nullness fix
Copilot Jul 22, 2025
e231164
Merge branch 'main' into copilot/fix-20e8251c-e9bc-4713-977e-9b992f4c…
T-Gro Jul 25, 2025
369234f
Move IEquatable nullness fix release note to FSharp.Compiler.Service
Copilot Jul 25, 2025
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
4 changes: 4 additions & 0 deletions .gitignore
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -137,3 +137,7 @@ positive.exe
*.bsl.actual
/src/FSharp.DependencyManager.Nuget/StandardError.txt
/src/FSharp.DependencyManager.Nuget/StandardOutput.txt

# Standard output/error files in root directory
StandardOutput.txt
StandardError.txt
1 change: 1 addition & 0 deletions docs/release-notes/.Language/preview.md
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -13,5 +13,6 @@
### Fixed

* Warn on uppercase identifiers in patterns. ([PR #15816](https://github.com/dotnet/fsharp/pull/15816))
* Fix nullness warnings when casting non-nullable values to `IEquatable<T>` to match C# behavior. ([Issue #18759](https://github.com/dotnet/fsharp/issues/18759))

### Changed
15 changes: 11 additions & 4 deletions src/Compiler/Checking/ConstraintSolver.fs
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -1407,11 +1407,17 @@ and SolveTypeEqualsTypeEqns csenv ndeep m2 trace cxsln origl1 origl2 =
ErrorD(ConstraintSolverTupleDiffLengths(csenv.DisplayEnv, csenv.eContextInfo, origl1, origl2, csenv.m, m2))
loop origl1 origl2

and SolveTypeEqualsTypeWithContravarianceEqns (csenv:ConstraintSolverEnv) ndeep m2 trace cxsln origl1 origl2 typars =
and SolveTypeEqualsTypeWithContravarianceEqns (csenv:ConstraintSolverEnv) ndeep m2 trace cxsln origl1 origl2 typars tyconRef =
let isContravariant (t:Typar) =
t.typar_opt_data
|> Option.map (fun d -> d.typar_is_contravariant)
|> Option.defaultValue(false)

// Special case for IEquatable<T>: treat its type parameter as contravariant for nullness purposes
// This matches C# behavior where IEquatable<T> is treated as contravariant for nullness, even though
// it's not formally marked as contravariant in IL.
// See: https://github.com/dotnet/fsharp/issues/18759 and https://github.com/dotnet/roslyn/issues/37187
let isIEquatableContravariantForNullness = tyconRefEq csenv.g tyconRef csenv.g.system_GenericIEquatable_tcref

match origl1, origl2, typars with
| [], [], [] -> CompleteD
Expand All @@ -1425,7 +1431,8 @@ and SolveTypeEqualsTypeWithContravarianceEqns (csenv:ConstraintSolverEnv) ndeep
let h1 =
// For contravariant typars (`<in T> in C#'), if the required type is WithNull, the actual type can have any nullness it wants
// Without this added logic, their nullness would be forced to be equal.
if isContravariant hTp && (nullnessOfTy csenv.g h2).TryEvaluate() = ValueSome NullnessInfo.WithNull then
// Special case: IEquatable<T> is treated as contravariant for nullness purposes to match C# behavior
if (isContravariant hTp || isIEquatableContravariantForNullness) && (nullnessOfTy csenv.g h2).TryEvaluate() = ValueSome NullnessInfo.WithNull then
replaceNullnessOfTy csenv.g.knownWithNull h1
else
h1
Expand Down Expand Up @@ -1534,11 +1541,11 @@ and SolveTypeSubsumesType (csenv: ConstraintSolverEnv) ndeep m2 (trace: Optional
(tyconRefEq g tagc1 g.byrefkind_In_tcr || tyconRefEq g tagc1 g.byrefkind_Out_tcr) ) -> ()
| _ -> return! SolveTypeEqualsType csenv ndeep m2 trace cxsln tag1 tag2
}
| _ -> SolveTypeEqualsTypeWithContravarianceEqns csenv ndeep m2 trace cxsln l1 l2 tc1.TyparsNoRange
| _ -> SolveTypeEqualsTypeWithContravarianceEqns csenv ndeep m2 trace cxsln l1 l2 tc1.TyparsNoRange tc1

| TType_app (tc1, l1, _) , TType_app (tc2, l2, _) when tyconRefEq g tc1 tc2 ->
trackErrors {
do! SolveTypeEqualsTypeWithContravarianceEqns csenv ndeep m2 trace cxsln l1 l2 tc1.TyparsNoRange
do! SolveTypeEqualsTypeWithContravarianceEqns csenv ndeep m2 trace cxsln l1 l2 tc1.TyparsNoRange tc1
do! SolveNullnessSubsumesNullness csenv m2 trace ty1 ty2 (nullnessOfTy g sty1) (nullnessOfTy g sty2)
}

Expand Down
15 changes: 14 additions & 1 deletion tests/FSharp.Compiler.ComponentTests/Language/Nullness/NullableReferenceTypesTests.fs
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -1540,4 +1540,17 @@ let main _ =
|> compile
//|> verifyIL ["abc"]
|> run
|> verifyOutputContains [|"Test true;,1 true,2 true,3 true,4 true,5 true,6 false,7 true,8 false,9 false,10 false,11 false,12 true"|]
|> verifyOutputContains [|"Test true;,1 true,2 true,3 true,4 true,5 true,6 false,7 true,8 false,9 false,10 false,11 false,12 true"|]

[<Fact>]
let ``No nullness warning when casting non-nullable to IEquatable`` () =
FSharp """module Test

open System

let x = ""
let y = x :> IEquatable<string> // Should not warn about nullness
"""
|> asLibrary
|> typeCheckWithStrictNullness
|> shouldSucceed
Loading