From aef7b930c0a5774ba78dc0e38a142e667f33a4af Mon Sep 17 00:00:00 2001
From: Tomas Grosup <tomasgrosup@microsoft.com>
Date: Thu, 7 Nov 2024 08:00:47 +0100
Subject: [PATCH] Fix missing nullness warning in case of multiple applicable
 candidates for method resolution (e.g. string and ROS<char>) (#17918)

* speculativeForoverloads exclusion was a mistake

* DateTime not annotated on desktop

* release notes
---
 .../.FSharp.Compiler.Service/9.0.200.md         |  1 +
 src/Compiler/Checking/ConstraintSolver.fs       |  3 +--
 .../Nullness/NullableReferenceTypesTests.fs     | 17 +++++++++++++++++
 3 files changed, 19 insertions(+), 2 deletions(-)

diff --git a/docs/release-notes/.FSharp.Compiler.Service/9.0.200.md b/docs/release-notes/.FSharp.Compiler.Service/9.0.200.md
index 0c604f1aa0d..360572c6093 100644
--- a/docs/release-notes/.FSharp.Compiler.Service/9.0.200.md
+++ b/docs/release-notes/.FSharp.Compiler.Service/9.0.200.md
@@ -9,6 +9,7 @@
 * Fix concurrency issue in `ILPreTypeDefImpl`  ([PR #17812](https://github.com/dotnet/fsharp/pull/17812))
 * Fix nullness inference for member val and other OO scenarios  ([PR #17845](https://github.com/dotnet/fsharp/pull/17845))
 * Fix internal error when analyzing incomplete inherit member ([PR #17905](https://github.com/dotnet/fsharp/pull/17905))
+* Fix missing nullness warning in case of method resolution multiple candidates ([PR #17917](https://github.com/dotnet/fsharp/pull/17918))
 
 ### Added
 
diff --git a/src/Compiler/Checking/ConstraintSolver.fs b/src/Compiler/Checking/ConstraintSolver.fs
index 6c79c33be97..6d59c68c7e8 100644
--- a/src/Compiler/Checking/ConstraintSolver.fs
+++ b/src/Compiler/Checking/ConstraintSolver.fs
@@ -1035,7 +1035,6 @@ and SolveTypMeetsTyparConstraints (csenv: ConstraintSolverEnv) ndeep m2 trace ty
 
 and shouldWarnUselessNullCheck (csenv:ConstraintSolverEnv) =
     csenv.g.checkNullness &&
-    csenv.IsSpeculativeForMethodOverloading = false &&
     csenv.SolverState.WarnWhenUsingWithoutNullOnAWithNullTarget.IsSome    
 
 // nullness1: actual
@@ -1102,7 +1101,7 @@ and SolveNullnessSubsumesNullness (csenv: ConstraintSolverEnv) m2 (trace: Option
         | NullnessInfo.WithNull, NullnessInfo.WithoutNull ->             
             CompleteD
         | NullnessInfo.WithoutNull, NullnessInfo.WithNull -> 
-            if csenv.g.checkNullness && not csenv.IsSpeculativeForMethodOverloading then               
+            if csenv.g.checkNullness then               
                  WarnD(ConstraintSolverNullnessWarningWithTypes(csenv.DisplayEnv, ty1, ty2, n1, n2, csenv.m, m2)) 
             else
                 CompleteD
diff --git a/tests/FSharp.Compiler.ComponentTests/Language/Nullness/NullableReferenceTypesTests.fs b/tests/FSharp.Compiler.ComponentTests/Language/Nullness/NullableReferenceTypesTests.fs
index 0da1169b9a8..2933d00e438 100644
--- a/tests/FSharp.Compiler.ComponentTests/Language/Nullness/NullableReferenceTypesTests.fs
+++ b/tests/FSharp.Compiler.ComponentTests/Language/Nullness/NullableReferenceTypesTests.fs
@@ -19,6 +19,23 @@ let typeCheckWithStrictNullness cu =
 
 
 
+[<FSharp.Test.FactForNETCOREAPPAttribute>]
+let ``Does report when null goes to DateTime Parse`` () =
+
+    FSharp """module TestLib
+open System
+let parsedDate = DateTime.Parse(null:(string|null))
+let parseDate2(s:string|null) = DateTime.Parse(s)
+let parsedDate3 = DateTime.Parse(null)
+    """
+    |> asLibrary
+    |> typeCheckWithStrictNullness
+    |> shouldFail
+    |> withDiagnostics     
+                [Error 3261, Line 3, Col 18, Line 3, Col 52, "Nullness warning: The types 'string' and 'string | null' do not have compatible nullability."
+                 Error 3261, Line 4, Col 33, Line 4, Col 50, "Nullness warning: The types 'string' and 'string | null' do not have compatible nullability."
+                 Error 3261, Line 5, Col 19, Line 5, Col 39, "Nullness warning: The type 'string' does not support 'null'."]
+
 [<Fact>]
 let ``Can convert generic value to objnull arg`` () =
     FSharp """module TestLib