Fix regression with var pattern nullability

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

@@ -208,7 +208,6 @@ protected override LocalState VisitSwitchStatementDispatch(BoundSwitchStatement
             // visit switch header
             Visit(node.Expression);
             var expressionState = ResultType;
-            var initialState = PossiblyConditionalState.Create(this);
 
             DeclareLocals(node.InnerLocals);
             foreach (var section in node.SwitchSections)
@@ -217,7 +216,7 @@ protected override LocalState VisitSwitchStatementDispatch(BoundSwitchStatement
                 DeclareLocals(section.Locals);
             }
 
-            var labelStateMap = LearnFromDecisionDag(node.Syntax, node.DecisionDag, node.Expression, expressionState, ref initialState);
+            var labelStateMap = LearnFromDecisionDag(node.Syntax, node.DecisionDag, node.Expression, expressionState);
             foreach (var section in node.SwitchSections)
             {
                 foreach (var label in section.SwitchLabels)
@@ -284,13 +283,11 @@ public PossiblyConditionalState Clone()
             }
         }
 
-        private PooledDictionary<LabelSymbol, (LocalState state, bool believedReachable)>
-            LearnFromDecisionDag(
+        private PooledDictionary<LabelSymbol, (LocalState state, bool believedReachable)> LearnFromDecisionDag(
             SyntaxNode node,
             BoundDecisionDag decisionDag,
             BoundExpression expression,
-            TypeWithState expressionType,
-            ref PossiblyConditionalState initialState)
+            TypeWithState expressionType)
         {
             // We reuse the slot at the beginning of a switch (or is-pattern expression), pretending that we are
             // not copying the input to evaluate the patterns.  In this way we infer non-nullability of the original
@@ -319,7 +316,7 @@ public PossiblyConditionalState Clone()
             tempMap.Add(rootTemp, (originalInputSlot, expressionType.Type));
 
             var nodeStateMap = PooledDictionary<BoundDecisionDagNode, (PossiblyConditionalState state, bool believedReachable)>.GetInstance();
-            nodeStateMap.Add(decisionDag.RootNode, (state: initialState.Clone(), believedReachable: true));
+            nodeStateMap.Add(decisionDag.RootNode, (state: PossiblyConditionalState.Create(this), believedReachable: true));
 
             var labelStateMap = PooledDictionary<LabelSymbol, (LocalState state, bool believedReachable)>.GetInstance();
 
@@ -735,8 +732,7 @@ private void VisitSwitchExpressionCore(BoundSwitchExpression node, bool inferTyp
 
             Visit(node.Expression);
             var expressionState = ResultType;
-            var state = PossiblyConditionalState.Create(this);
-            var labelStateMap = LearnFromDecisionDag(node.Syntax, node.DecisionDag, node.Expression, expressionState, ref state);
+            var labelStateMap = LearnFromDecisionDag(node.Syntax, node.DecisionDag, node.Expression, expressionState);
             var endState = UnreachableState();
 
             if (!node.ReportedNotExhaustive && node.DefaultLabel != null &&
@@ -846,8 +842,7 @@ public override BoundNode VisitIsPatternExpression(BoundIsPatternExpression node
             VisitPatternForRewriting(node.Pattern);
             Visit(node.Expression);
             var expressionState = ResultType;
-            var state = PossiblyConditionalState.Create(this);
-            var labelStateMap = LearnFromDecisionDag(node.Syntax, node.DecisionDag, node.Expression, expressionState, ref state);
+            var labelStateMap = LearnFromDecisionDag(node.Syntax, node.DecisionDag, node.Expression, expressionState);
             var trueState = labelStateMap.TryGetValue(node.IsNegated ? node.WhenFalseLabel : node.WhenTrueLabel, out var s1) ? s1.state : UnreachableState();
             var falseState = labelStateMap.TryGetValue(node.IsNegated ? node.WhenTrueLabel : node.WhenFalseLabel, out var s2) ? s2.state : UnreachableState();
             labelStateMap.Free();

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

@@ -146719,5 +146719,71 @@ void M(I3 i)
             AssertEx.Equal(new string[] { "I<object?>", "I2<object!>", "I<object!>" },
                 i3.AllInterfaces().ToTestDisplayStrings(TypeWithAnnotations.TestDisplayFormat));
         }
+
+        [Fact]
+        public void VarPatternDeclaration_TopLevel()
+        {
+            var src = @"
+#nullable enable
+public class C
+{
+    public void M(string? x)
+    {
+        if (Identity(x) is var y)
+        {
+            y.ToString(); // 1
+        }
+
+        if (Identity(x) is not null and var z)
+        {
+            z.ToString();
+        }
+    }
+
+    public T Identity<T>(T t) => t;
+}
+";
+            var comp = CreateCompilation(src);
+            comp.VerifyDiagnostics(
+                // (9,13): warning CS8602: Dereference of a possibly null reference.
+                //             y.ToString(); // 1
+                Diagnostic(ErrorCode.WRN_NullReferenceReceiver, "y").WithLocation(9, 13)
+                );
+        }
+
+        [Fact]
+        public void VarPatternDeclaration_Nested()
+        {
+            var src = @"
+#nullable enable
+public class Container<T>
+{
+    public T Item { get; set; } = default!;
+}
+public class C
+{
+    public void M(Container<string?> x)
+    {
+        if (Identity(x) is { Item: var y })
+        {
+            y.ToString(); // 1
+        }
+
+        if (Identity(x) is { Item: not null and var z })
+        {
+            z.ToString();
+        }
+    }
+
+    public T Identity<T>(T t) => t;
+}
+";
+            var comp = CreateCompilation(src);
+            comp.VerifyDiagnostics(
+                // (13,13): warning CS8602: Dereference of a possibly null reference.
+                //             y.ToString(); // 1
+                Diagnostic(ErrorCode.WRN_NullReferenceReceiver, "y").WithLocation(13, 13)
+                );
+        }
     }
 }