Ensure that IOperations nodes within an IsPatternExpression are part of the Operation tree

Fixes #19790

src/Compilers/CSharp/Portable/BoundTree/Expression.cs

@@ -2820,6 +2820,8 @@ public override void Accept(OperationVisitor visitor)
 
         // TODO: implement IOperation for pattern-matching constructs (https://github.com/dotnet/roslyn/issues/8699)
         protected override OperationKind ExpressionKind => OperationKind.None;
+
+        protected override ImmutableArray<IOperation> Children => ImmutableArray.Create<IOperation>(this.Expression, this.Pattern);
     }
 
     /// <summary>

src/Compilers/CSharp/Test/Semantic/IOperation/IOperationTests_IParameterReferenceExpression.cs

@@ -827,5 +827,28 @@ static void M(int x, __arglist)
 
             VerifyOperationTreeAndDiagnosticsForTest<InvocationExpressionSyntax>(source, expectedOperationTree, expectedDiagnostics);
         }
+
+        [Fact, WorkItem(19790, "https://github.com/dotnet/roslyn/issues/19790")]
+        public void ParameterReference_IsPatternExpression()
+        {
+            string source = @"
+class Class1
+{
+    public void Method1(object x)
+    {
+        if (/*<bind>*/x is int y/*</bind>*/) { }
+    }
+}
+";
+            string expectedOperationTree = @"
+IOperation:  (OperationKind.None) (Syntax: 'x is int y')
+  Children(2): IParameterReferenceExpression: x (OperationKind.ParameterReferenceExpression, Type: System.Object) (Syntax: 'x')
+    IOperation:  (OperationKind.None) (Syntax: 'int y')
+      Children(1): ILocalReferenceExpression: y (OperationKind.LocalReferenceExpression, Type: System.Int32) (Syntax: 'int y')
+";
+            var expectedDiagnostics = DiagnosticDescription.None;
+
+            VerifyOperationTreeAndDiagnosticsForTest<IsPatternExpressionSyntax>(source, expectedOperationTree, expectedDiagnostics);
+        }
     }
 }
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