NullableWalker.VisitSwitchStatementDispatch - visit local declaration…

…s before the switch expression, which can cause us to "split" the state. (#56031)

Fixes #55227.

src/Compilers/CSharp/Portable/FlowAnalysis/NullableWalker_Patterns.cs

@@ -228,17 +228,17 @@ protected override LocalState VisitSwitchStatementDispatch(BoundSwitchStatement
                 }
             }
 
-            // visit switch header
-            Visit(node.Expression);
-            var expressionState = ResultType;
-
             DeclareLocals(node.InnerLocals);
             foreach (var section in node.SwitchSections)
             {
                 // locals can be alive across jumps in the switch sections, so we declare them early.
                 DeclareLocals(section.Locals);
             }
 
+            // visit switch header
+            Visit(node.Expression);
+            var expressionState = ResultType;
+
             var labelStateMap = LearnFromDecisionDag(node.Syntax, node.DecisionDag, node.Expression, expressionState, stateWhenNotNullOpt: null);
             foreach (var section in node.SwitchSections)
             {

src/Compilers/CSharp/Test/Semantic/Semantics/NullableReferenceTypesTests.cs

@@ -152149,5 +152149,63 @@ ref T Passthrough(ref T value)
                 Diagnostic(ErrorCode.WRN_NullabilityMismatchInAssignment, "value2").WithArguments("T?", "T").WithLocation(9, 20)
                 );
         }
+
+        [Fact, WorkItem(55227, "https://github.com/dotnet/roslyn/issues/55227")]
+        public void Issue55227_01()
+        {
+            var source = @"
+#nullable enable
+
+static class Program
+{
+    public static void Main()
+    {
+    }
+
+    public static object Test(object a)
+    {
+        var isNullCheck = new global::System.Func<object, object>((p) =>
+        {
+            switch (p == null)
+            {
+                case var isnull:
+                    return isnull;
+            }
+        });
+        return isNullCheck(a);
+    }
+}
+";
+
+            CreateCompilation(source).VerifyDiagnostics();
+        }
+
+        [Fact, WorkItem(55227, "https://github.com/dotnet/roslyn/issues/55227")]
+        public void Issue55227_02()
+        {
+            var source = @"
+#nullable enable
+
+static class Program
+{
+    public static void Main()
+    {
+    }
+
+    public static object Test(object a)
+    {
+        var isNullCheck = new global::System.Func<object, object>((p) =>
+            (p == null) switch 
+            {
+                var isnull => isnull
+            }
+        );
+        return isNullCheck(a);
+    }
+}
+";
+
+            CreateCompilation(source).VerifyDiagnostics();
+        }
     }
 }