Avoid asserting unvisited nested binary operators

src/Compilers/CSharp/Portable/Binder/Binder.ValueChecks.cs

@@ -4055,6 +4055,14 @@ internal uint GetValEscape(BoundExpression expr, uint scopeOfTheContainingExpres
                 case BoundKind.BinaryOperator:
                     var binary = (BoundBinaryOperator)expr;
 
+#if DEBUG
+                    // Nested binary operators on the left-hand side are not visited by BoundTreeWalkerWithStackGuardWithoutRecursionOnTheLeftOfBinaryOperator.
+                    if (binary.Left is BoundBinaryOperator)
+                    {
+                        _visited.Add(binary.Left);
+                    }
+#endif
+
                     if (binary.Method is { } binaryMethod)
                     {
                         return GetInvocationEscapeScope(
@@ -4734,6 +4742,14 @@ internal bool CheckValEscape(SyntaxNode node, BoundExpression expr, uint escapeF
                         return true;
                     }
 
+#if DEBUG
+                    // Nested binary operators on the left-hand side are not visited by BoundTreeWalkerWithStackGuardWithoutRecursionOnTheLeftOfBinaryOperator.
+                    if (binary.Left is BoundBinaryOperator)
+                    {
+                        _visited.Add(binary.Left);
+                    }
+#endif
+
                     if (binary.Method is { } binaryMethod)
                     {
                         return CheckInvocationEscape(

src/Compilers/CSharp/Portable/Binder/RefSafetyAnalysis.cs

@@ -283,7 +283,7 @@ private void AssertVisited(BoundExpression expr)
             }
             else if (_visited is { } && _visited.Count <= MaxTrackVisited)
             {
-                Debug.Assert(_visited.Contains(expr));
+                Debug.Assert(_visited.Contains(expr), $"Expected {expr} `{expr.Syntax}` to be visited.");
             }
         }
 #endif

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

@@ -7871,6 +7871,37 @@ ref struct S
                 Diagnostic(ErrorCode.ERR_EscapeVariable, "s").WithArguments("s").WithLocation(47, 16));
         }
 
+        [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/71773")]
+        public void UserDefinedBinaryOperator_RefStruct_Nested()
+        {
+            var source = """
+                class C
+                {
+                    S M()
+                    {
+                        S s;
+                        s = default(S) + 100 + 200;
+                        return s;
+                    }
+                }
+
+                ref struct S
+                {
+                    public static S operator+(S y, in int x) => throw null;
+                }
+                """;
+            CreateCompilation(source).VerifyDiagnostics(
+                // (6,13): error CS8347: Cannot use a result of 'S.operator +(S, in int)' in this context because it may expose variables referenced by parameter 'x' outside of their declaration scope
+                //         s = default(S) + 100 + 200;
+                Diagnostic(ErrorCode.ERR_EscapeCall, "default(S) + 100").WithArguments("S.operator +(S, in int)", "x").WithLocation(6, 13),
+                // (6,13): error CS8347: Cannot use a result of 'S.operator +(S, in int)' in this context because it may expose variables referenced by parameter 'y' outside of their declaration scope
+                //         s = default(S) + 100 + 200;
+                Diagnostic(ErrorCode.ERR_EscapeCall, "default(S) + 100 + 200").WithArguments("S.operator +(S, in int)", "y").WithLocation(6, 13),
+                // (6,26): error CS8156: An expression cannot be used in this context because it may not be passed or returned by reference
+                //         s = default(S) + 100 + 200;
+                Diagnostic(ErrorCode.ERR_RefReturnLvalueExpected, "100").WithLocation(6, 26));
+        }
+
         [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/71773")]
         public void UserDefinedBinaryOperator_RefStruct_Scoped_Left()
         {
@@ -8531,5 +8562,16 @@ static R F2()
                 //         return new R(1) | new R(2);
                 Diagnostic(ErrorCode.ERR_RefReturnLvalueExpected, "1").WithLocation(18, 22));
         }
+
+        [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/72873")]
+        public void Utf8Addition()
+        {
+            var code = """
+                using System;
+                ReadOnlySpan<byte> x = "Hello"u8 + " "u8 + "World!"u8;
+                Console.WriteLine(x.Length);
+                """;
+            CreateCompilation(code, targetFramework: TargetFramework.Net70).VerifyDiagnostics();
+        }
     }
 }