NullableWalker.DebugVerifier: Explicitly visit BoundCollectionExpressionSpreadElement.Conversion

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

@@ -147,6 +147,18 @@ private void VerifyExpression(BoundExpression expression, bool overrideSkippedEx
                 return null;
             }
 
+            public override BoundNode? VisitCollectionExpressionSpreadElement(BoundCollectionExpressionSpreadElement node)
+            {
+                Visit(node.Expression);
+
+                if (node.Conversion is BoundConversion conversion)
+                {
+                    Visit(conversion);
+                }
+
+                return null;
+            }
+
             public override BoundNode? VisitDeconstructionAssignmentOperator(BoundDeconstructionAssignmentOperator node)
             {
                 // https://github.com/dotnet/roslyn/issues/35010: handle

src/Compilers/CSharp/Test/Emit3/Semantics/CollectionExpressionTests.cs

@@ -47326,5 +47326,38 @@ .locals init (int V_0, //i
             Assert.Contains(libraryRef, usedRefs);
 
         }
+
+        [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/81552")]
+        public void VisitedNodes_CollectionExpression()
+        {
+            var source = """
+                #nullable enable
+                using System;
+                using System.Collections;
+                using System.Collections.Generic;
+                using System.Runtime.CompilerServices;
+
+                Merge([.. D.P]);
+
+                static void Merge(params ReadOnlySpan<C> cs) { }
+
+                [CollectionBuilder(typeof(C), methodName: nameof(Create))]
+                class C : IEnumerable<int>
+                {
+                    public static C Create(ReadOnlySpan<int> span) => new();
+
+                    public IEnumerator<int> GetEnumerator() => throw new NotImplementedException();
+
+                    IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
+                }
+
+                class D
+                {
+                    public static IEnumerable<int> P => [1];
+                }
+                """;
+
+            CompileAndVerify(CreateCompilationWithSpan([source, CollectionBuilderAttributeDefinition]), expectedOutput: "").VerifyDiagnostics();
+        }
     }
 }