Fix RCS1202 - check if expr flow state is not null (#1542)

ChangeLog.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Fixed
+
+- Fix analyzer [RCS1202](https://josefpihrt.github.io/docs/roslynator/analyzers/RCS1202) ([PR](https://github.com/dotnet/roslynator/pull/1542))
+
 ## [4.12.6] - 2024-09-23
 
 ### Added

src/Analyzers/CSharp/Analysis/AvoidNullReferenceExceptionAnalyzer.cs

@@ -198,6 +198,9 @@ private static void AnalyzeAsExpression(SyntaxNodeAnalysisContext context)
         if (topExpression is null)
             return;
 
+        if (semanticModel.GetTypeInfo(expression, cancellationToken).Nullability.FlowState == NullableFlowState.NotNull)
+            return;
+
         if (semanticModel
             .GetTypeSymbol(asExpression, cancellationToken)?
             .IsReferenceType != true)

src/Tests/Analyzers.Tests/RCS1202AvoidNullReferenceExceptionTests2.cs

@@ -146,6 +146,34 @@ public void M()
         (this as I).M();
     }
 }
+");
+    }
+
+    [Fact, Trait(Traits.Analyzer, DiagnosticIdentifiers.AvoidNullReferenceException)]
+    public async Task TestNoDiagnostic_ExpressionIsDefinitelyNotNull()
+    {
+        await VerifyNoDiagnosticAsync(@"
+#nullable enable
+
+public interface I
+{
+    void M() { }
+}
+
+public class P : I;
+
+public class C
+{
+    public required P P { get; set; }
+
+    public void M()
+    {
+        if (this.P is not null)
+        {
+            (this.P as I).M();
+        }
+    }
+}
 ");
     }
 }