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

Author: Copilot

.gitignore

@@ -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

docs/release-notes/.FSharp.Compiler.Service/10.0.100.md

@@ -13,6 +13,7 @@
 * Shorthand lambda: fix completion for chained calls and analysis for unfinished expression ([PR #18560](https://github.com/dotnet/fsharp/pull/18560))
 * Completion: fix previous namespace considered opened [PR #18609](https://github.com/dotnet/fsharp/pull/18609)
 * Fix active pattern typechecking regression. ([Issue #18638](https://github.com/dotnet/fsharp/issues/18638), [PR #18642](https://github.com/dotnet/fsharp/pull/18642))
+* 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
 * Use `errorR` instead of `error` in `CheckDeclarations.fs` when possible. ([PR #18645](https://github.com/dotnet/fsharp/pull/18645))

src/Compiler/Checking/ConstraintSolver.fs

@@ -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
@@ -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
@@ -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)
             }
 

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

@@ -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