'is' operator should not affect StateWhenFalse (#30283)

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

@@ -4001,7 +4001,6 @@ public override BoundNode VisitIsOperator(BoundIsOperator node)
             {
                 Split();
                 this.StateWhenTrue[slot] = true;
-                this.StateWhenFalse[slot] = false;
             }
 
             SetResult(node);

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

@@ -49,6 +49,86 @@ static void Main()
                 );
         }
 
+        [Fact, WorkItem(29318, "https://github.com/dotnet/roslyn/issues/29318")]
+        public void IsOperatorOnNonNullExpression()
+        {
+            var source = @"
+class C
+{
+    void M(object o)
+    {
+        if (o is string)
+        {
+            o.ToString();
+        }
+        else
+        {
+            o.ToString();
+        }
+    }
+}
+";
+
+            var c = CreateCompilation(new[] { source, NonNullTypesTrue }, parseOptions: TestOptions.Regular8);
+            c.VerifyDiagnostics();
+        }
+
+        [Fact, WorkItem(29318, "https://github.com/dotnet/roslyn/issues/29318")]
+        public void IsOperatorOnMaybeNullExpression()
+        {
+            var source = @"
+class C
+{
+    static void Main(object? o)
+    {
+        if (o is string)
+        {
+            o.ToString();
+        }
+        else
+        {
+            o.ToString(); // 1
+        }
+    }
+}
+";
+
+            var c = CreateCompilation(new[] { source, NonNullTypesTrue }, parseOptions: TestOptions.Regular8);
+            c.VerifyDiagnostics(
+                // (12,13): warning CS8602: Possible dereference of a null reference.
+                //             o.ToString(); // 1
+                Diagnostic(ErrorCode.WRN_NullReferenceReceiver, "o").WithLocation(12, 13)
+                );
+        }
+
+        [Fact, WorkItem(29318, "https://github.com/dotnet/roslyn/issues/29318")]
+        public void IsOperatorOnUnconstrainedType()
+        {
+            var source = @"
+class C
+{
+    static void M<T>(T t)
+    {
+        if (t is string)
+        {
+            t.ToString();
+        }
+        else
+        {
+            t.ToString(); // 1
+        }
+    }
+}
+";
+
+            var c = CreateCompilation(new[] { source, NonNullTypesTrue }, parseOptions: TestOptions.Regular8);
+            c.VerifyDiagnostics(
+                // (12,13): warning CS8602: Possible dereference of a null reference.
+                //             t.ToString(); // 1
+                Diagnostic(ErrorCode.WRN_NullReferenceReceiver, "t").WithLocation(12, 13)
+                );
+        }
+
         [Fact]
         public void OmittedCall()
         {