Fix nullness flow for type aliases after null pattern (#18852)

Author: Copilot

docs/release-notes/.FSharp.Compiler.Service/11.0.0.md

@@ -1,5 +1,6 @@
 ### Fixed
 
+* Nullness: Fix nullness refinement in match expressions to correctly narrow type to non-null after matching null case. ([Issue #18488](https://github.com/dotnet/fsharp/issues/18488), [PR #18852](https://github.com/dotnet/fsharp/pull/18852))
 * Scripts: Fix resolving the dotnet host path when an SDK directory is specified. ([PR #18960](https://github.com/dotnet/fsharp/pull/18960))
 * Fix excessive StackGuard thread jumping ([PR #18971](https://github.com/dotnet/fsharp/pull/18971))
 * Adjust conservative method-overload duplicate detection rules for nativeptr types ([PR #18911](https://github.com/dotnet/fsharp/pull/18911))

src/Compiler/Checking/Expressions/CheckExpressions.fs

@@ -10761,6 +10761,9 @@ and TcMatchClause cenv inputTy (resultTy: OverallTy) env isFirst tpenv synMatchC
 
     let inputTypeForNextPatterns=
         let removeNull t =
+            // Strip type equations (including abbreviations) and set nullness to non-null.
+            // For type abbreviations like `type objnull = obj | null`, we need to expand
+            // the abbreviation and apply non-null to the underlying type.
             let stripped = stripTyEqns cenv.g t
             replaceNullnessOfTy KnownWithoutNull stripped
         let rec isWild (p:Pattern) =

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

@@ -1554,3 +1554,72 @@ let y = x :> IEquatable<string> // Should not warn about nullness
     |> asLibrary
     |> typeCheckWithStrictNullness
     |> shouldSucceed
+
+// Regression for https://github.com/dotnet/fsharp/issues/18488
+// This tests the exact scenario from the issue with obj | null return type
+[<FSharp.Test.FactForNETCOREAPPAttribute>]
+let ``Match null branch should refine variable to non-null - exact issue scenario`` () =
+    FSharp """module Test
+
+let bar : obj =
+    let getEnvironmentVariable : string -> (obj|null) = fun _ -> null
+
+    match "ENVVAR" |> getEnvironmentVariable with
+    | null -> obj() // return some obj in null case
+    | x -> x // x should be obj, not obj | null - no warning expected
+    """
+    |> asLibrary
+    |> typeCheckWithStrictNullness
+    |> shouldSucceed
+
+// Regression for https://github.com/dotnet/fsharp/issues/18488
+[<FSharp.Test.FactForNETCOREAPPAttribute>]
+let ``Match null branch should refine variable to non-null in subsequent branches - type alias`` () =
+    FSharp """module Test
+
+// Type alias for nullable obj
+type objnull = obj | null
+
+let getEnvAliasObj (_: string) : objnull = failwith "stub"
+
+// After matching null case, x should be refined to non-null
+let valueAliasObj =
+    match getEnvAliasObj "ENVVAR" with
+    | null -> "missing"
+    | x -> x.GetType().Name // x should be obj, not obj | null - no warning expected
+    """
+    |> asLibrary
+    |> typeCheckWithStrictNullness
+    |> shouldSucceed
+
+// Regression for https://github.com/dotnet/fsharp/issues/18488
+[<FSharp.Test.FactForNETCOREAPPAttribute>]
+let ``Match null branch should refine variable to non-null in subsequent branches - direct nullable type`` () =
+    FSharp """module Test
+
+let getValue () : string | null = failwith "stub"
+
+// After matching null case, x should be refined to non-null
+let result =
+    match getValue () with
+    | null -> "missing"
+    | x -> x.ToUpper() // x should be string, not string | null - no warning expected
+    """
+    |> asLibrary
+    |> typeCheckWithStrictNullness
+    |> shouldSucceed
+
+// Regression for https://github.com/dotnet/fsharp/issues/18488
+[<FSharp.Test.FactForNETCOREAPPAttribute>]
+let ``Match null branch should refine variable to non-null - Environment.GetEnvironmentVariable`` () =
+    FSharp """module Test
+
+// Real-world scenario from issue #18488
+let value =
+    match System.Environment.GetEnvironmentVariable "ENVVAR" with
+    | null -> "missing"
+    | x -> x.ToLower() // x should be string, not string | null - no warning expected
+    """
+    |> asLibrary
+    |> typeCheckWithStrictNullness
+    |> shouldSucceed

tests/ILVerify/ilverify.ps1

@@ -27,10 +27,12 @@ Write-Host "Checking whether running on Windows: $IsWindows"
 Write-Host "Repository path: $repo_path"
 
 [string] $script = if ($IsWindows) { Join-Path $repo_path "build.cmd" } else { Join-Path $repo_path "build.sh" }
-[string] $additional_arguments = if ($IsWindows) { "-noVisualStudio" } else { "" }
+[string] $additional_arguments = if ($IsWindows) { "-noVisualStudio -ci -bootstrap" } else { "" }
 
 # Set environment variable to disable UpdateXlf target (not needed for IL verification)
 $env:UpdateXlfOnBuild = "false"
+# Disable PDB conversion for SymStore (not needed for IL verification)
+$env:PublishWindowsPdb = "false"
 
 # Set configurations to build
 [string[]] $configurations = @("Debug", "Release")