Regression test for 32503

This issue is fixed. Adding a regression test.

closes #32503

.gitignore

@@ -24,6 +24,9 @@ x64/
 # Enable "build/" folder in the NuGet Packages folder since NuGet packages use it for MSBuild targets
 !packages/*/build/
 
+# Debug artifacts
+launchSettings.json
+
 # Prevent accidental re-checkin of NuGet.exe
 NuGet.exe
 
@@ -191,4 +194,4 @@ $RECYCLE.BIN/
 .idea/
 
 # WPF temp projects
-*wpftmp.*
\ No newline at end of file
+*wpftmp.*

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

@@ -74820,5 +74820,26 @@ void M1(C? c1)
                 //         c1 ??= M2(c1);
                 Diagnostic(ErrorCode.WRN_NullReferenceArgument, "c1").WithArguments("c1", "C C.M2(C c1)").WithLocation(6, 19));
         }
+
+        [WorkItem(32503, "https://github.com/dotnet/roslyn/issues/32503")]
+        [Fact]
+        public void PatternDeclarationBreaksNullabeAnalysis()
+        {
+            var source = @"
+#nullable enable
+class A { }
+class B : A
+{
+    A M()
+    {
+        var s = new A();
+        if (s is B b) {}
+        return s; 
+    }
+} 
+";
+            var comp = CreateCompilation(source);
+            comp.VerifyDiagnostics();
+        }
     }
 }