Fix nullness flow for type aliases after null pattern - targeted approach

Copilot · T-Gro · Copilot · commit 6ebd419a4db6 · 2025-09-08T15:00:39.000Z
Co-authored-by: T-Gro &lt;46543583+T-Gro@users.noreply.github.com&gt;
diff --git a/src/Compiler/Checking/Expressions/CheckExpressions.fs b/src/Compiler/Checking/Expressions/CheckExpressions.fs
@@ -10733,18 +10733,15 @@ and TcMatchClause cenv inputTy (resultTy: OverallTy) env isFirst tpenv synMatchC
 
     let inputTypeForNextPatterns=
         let removeNull t =
-            // Preserve original type structure while refining nullness  
-            match stripTyEqns cenv.g t with
-            | TType_app (tcref, _, _) when not tcref.Deref.IsStructOrEnumTycon ->
-                // Apply to original type to preserve aliases
-                match t with
-                | TType_app (tcrefOrig, tinstOrig, _) -> TType_app (tcrefOrig, tinstOrig, KnownWithoutNull)
-                | _ -> replaceNullnessOfTy KnownWithoutNull t
-            | TType_var _ ->
-                match t with  
-                | TType_var (tpOrig, _) -> TType_var (tpOrig, KnownWithoutNull)
-                | _ -> replaceNullnessOfTy KnownWithoutNull t
-            | _ -> t
+            // Check if this is a type abbreviation that we should preserve
+            match t with
+            | TType_app (tcref, tinst, _) when tcref.Deref.IsTypeAbbrev ->
+                // Preserve the type abbreviation structure while refining nullness
+                TType_app (tcref, tinst, KnownWithoutNull)
+            | _ ->
+                // Use existing logic for non-abbreviation types
+                let stripped = stripTyEqns cenv.g t
+                replaceNullnessOfTy KnownWithoutNull stripped
         let rec isWild (p:Pattern) =
             match p with
             | TPat_wild _ -> true
diff --git a/src/Compiler/TypedTree/TypedTreeOps.fs b/src/Compiler/TypedTree/TypedTreeOps.fs
@@ -782,27 +782,6 @@ let rec stripTyEqnsA g canShortcut ty =
 
 let stripTyEqns g ty = stripTyEqnsA g false ty
 
-/// Try to refine a type by removing 'null' from its top-level nullness, preserving any type abbreviations.
-/// - Strip type equations/abbreviations only for the purpose of deciding if we can remove 'null'.
-/// - If applicable, apply the refinement to the original 'ty' using replaceNullnessOfTy, so aliases are not discarded.
-/// - Only refine reference-like heads (including type variables).
-let tryRefineToNonNullPreservingAbbrev (g: TcGlobals) (ty: TType) : TType option =
-    // Use stripTyEqns to decide if we can refine, but apply to original type
-    let stripped = stripTyEqns g ty
-    match stripped with
-    | TType_app (tcref, _, _) when not tcref.Deref.IsStructOrEnumTycon ->
-        // Apply refinement to original type structure to preserve aliases
-        match ty with
-        | TType_app (tcrefOrig, tinstOrig, _) -> Some (TType_app (tcrefOrig, tinstOrig, KnownWithoutNull))
-        | TType_var (tpOrig, _) -> Some (TType_var (tpOrig, KnownWithoutNull))
-        | TType_fun (dOrig, rOrig, _) -> Some (TType_fun (dOrig, rOrig, KnownWithoutNull))
-        | _ -> Some (replaceNullnessOfTy KnownWithoutNull ty)
-    | TType_var _ ->
-        match ty with
-        | TType_var (tpOrig, _) -> Some (TType_var (tpOrig, KnownWithoutNull))
-        | _ -> Some (replaceNullnessOfTy KnownWithoutNull ty)
-    | _ -> None
-
 let evalTupInfoIsStruct aexpr = 
     match aexpr with 
     | TupInfo.Const b -> b
diff --git a/src/Compiler/TypedTree/TypedTreeOps.fsi b/src/Compiler/TypedTree/TypedTreeOps.fsi
@@ -609,12 +609,6 @@ val stripTyEqnsA: TcGlobals -> bool -> TType -> TType
 
 val stripTyEqns: TcGlobals -> TType -> TType
 
-/// Try to refine a type by removing 'null' from its top-level nullness, preserving any type abbreviations.
-/// - Strip type equations/abbreviations only for the purpose of deciding if we can remove 'null'.
-/// - If applicable, apply the refinement to the original 'ty' using replaceNullnessOfTy, so aliases are not discarded.
-/// - Only refine reference-like heads (including type variables).
-val tryRefineToNonNullPreservingAbbrev: TcGlobals -> TType -> TType option
-
 val stripTyEqnsAndMeasureEqns: TcGlobals -> TType -> TType
 
 val tryNormalizeMeasureInType: TcGlobals -> TType -> TType
diff --git a/tests/FSharp.Compiler.ComponentTests/Language/Nullness/Match_Null_DefaultingAndAlias.fs b/tests/FSharp.Compiler.ComponentTests/Language/Nullness/Match_Null_DefaultingAndAlias.fs
diff --git a/tests/FSharp.Compiler.ComponentTests/Language/Nullness/NullableReferenceTypesTests.fs b/tests/FSharp.Compiler.ComponentTests/Language/Nullness/NullableReferenceTypesTests.fs
@@ -1554,38 +1554,3 @@ let y = x :> IEquatable<string> // Should not warn about nullness
     |> asLibrary
     |> typeCheckWithStrictNullness
     |> shouldSucceed
-
-[<Fact>]
-let ``Match null pattern refines type to non-null preserving aliases`` () =
-    FSharp """module Test
-
-type objnull = obj | null
-type stringnull = string | null
-
-// 1) Defaulting case: result unconstrained; null pattern forces a nullable top type.
-let getEnvDefault (_: string) = failwith ""
-
-let valueDefault =
-    match "ENVVAR" |> getEnvDefault with
-    | null -> "missing"
-    | x -> x.ToString() // x must be refined to obj (non-null)
-
-// 2) Alias to obj | null
-let getEnvAliasObj (_: string) : objnull = failwith "stub"
-
-let valueAliasObj =
-    match getEnvAliasObj "ENVVAR" with
-    | null -> "missing"
-    | x -> x.ToString() // x must be refined to obj (non-null)
-
-// 3) Alias to string | null
-let getEnvAliasStr (_: string) : stringnull = failwith "stub"
-
-let valueAliasStr =
-    match getEnvAliasStr "ENVVAR" with
-    | null -> 0
-    | s -> s.Length // s must be refined to string (non-null)
-    """
-    |> withNullnessOptions
-    |> typecheck
-    |> shouldSucceed
