Preserve type aliases in match | null refinement

docs/release-notes/.FSharp.Compiler.Service/11.0.100.md

@@ -1,5 +1,6 @@
 ### Fixed
 
+* Preserve type abbreviations (`string`, user-defined aliases) in the refined type of bindings introduced after a `| null` pattern in a `match` expression. ([Issue #19646](https://github.com/dotnet/fsharp/issues/19646), [PR #19745](https://github.com/dotnet/fsharp/pull/19745))
 * Fix attributes on return type of unparenthesized tuple methods being silently dropped from IL. ([Issue #462](https://github.com/dotnet/fsharp/issues/462), [PR #19714](https://github.com/dotnet/fsharp/pull/19714))
 * Fix internal error FS0073 "Undefined or unsolved type variable" in IlxGen when nested inline SRTP functions with multiple overloads leave unsolved typars in the non-witness codegen path. ([Issue #19709](https://github.com/dotnet/fsharp/issues/19709), [PR #19710](https://github.com/dotnet/fsharp/pull/19710))
 * Fix NRE when calling virtual Object methods on value types through inline SRTP functions. ([Issue #8098](https://github.com/dotnet/fsharp/issues/8098), [PR #19511](https://github.com/dotnet/fsharp/pull/19511))

src/Compiler/Checking/Expressions/CheckExpressions.fs

@@ -10969,11 +10969,18 @@ 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
+            // Remove nullness, preserving aliases when possible (#19646):
+            //   `string | null`  → `string`  (not `System.String`)
+            //   `MyStr | null`   → `MyStr`
+            // Fall back to stripping when nullness is baked into the abbreviation
+            // RHS, e.g. `type objnull = obj | null` (#18488).
+            let nonNullOriginal = replaceNullnessOfTy KnownWithoutNull t
+            match (nullnessOfTy cenv.g nonNullOriginal).TryEvaluate() with
+            | ValueSome NullnessInfo.WithoutNull ->
+                nonNullOriginal
+            | _ ->
+                let stripped = stripTyEqns cenv.g t
+                replaceNullnessOfTy KnownWithoutNull stripped
         let rec isWild (p:Pattern) =
             match p with
             | TPat_wild _ -> true

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

@@ -2373,3 +2373,25 @@ let main _ = 0
     |> compile
     |> run
     |> verifyOutputContains [|"-1"|]
+
+// https://github.com/dotnet/fsharp/issues/19646
+// After `| null -> … | s -> …`, `s` must keep its type alias, not the BCL type.
+[<Theory>]
+[<InlineData("", "string", "'string'")>]
+[<InlineData("type MyStr = string", "MyStr", "'MyStr'")>]
+[<InlineData("open System\ntype MyUri = Uri", "MyUri", "'MyUri'")>]
+let ``Issue 19646 - type alias is preserved after null pattern``
+    (typeDef: string, paramTypeName: string, expectedSubstring: string) =
+    FSharp $"""module Test
+
+{typeDef}
+
+let test (x: {paramTypeName} | null) : int =
+    match x with
+    | null -> 0
+    | s -> s
+    """
+    |> asLibrary
+    |> typeCheckWithStrictNullness
+    |> shouldFail
+    |> withDiagnosticMessageMatches expectedSubstring