Handle early attribute required members cycle

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

@@ -258,14 +258,12 @@ private BoundAttribute BindAttributeCore(AttributeSyntax node, NamedTypeSymbol a
             Debug.Assert(boundConstructorArguments.All(a => !a.NeedsToBeConverted()));
 
             ImmutableArray<string?> boundConstructorArgumentNamesOpt = analyzedArguments.ConstructorArguments.GetNames();
+            // We delay reporting required member errors because:
+            // 1. If we didn't do it lazily during early attribute binding, we'd cause a cycle.
+            // 2. If we do it lazily during early attribute binding, we force every early-bound attribute to rebind later because the lazy diagnostic can't be computed yet.
+            // So instead, we report the error in `LoadAndValidateAttributes` after all attributes have been bound.
             ImmutableArray<BoundAssignmentOperator> boundNamedArguments = analyzedArguments.NamedArguments?.ToImmutableAndFree() ?? ImmutableArray<BoundAssignmentOperator>.Empty;
             Debug.Assert(boundNamedArguments.All(arg => !arg.Right.NeedsToBeConverted()));
-
-            if (attributeConstructor is not null)
-            {
-                CheckRequiredMembersInObjectInitializer(attributeConstructor, ImmutableArray<BoundExpression>.CastUp(boundNamedArguments), node, diagnostics);
-            }
-
             analyzedArguments.ConstructorArguments.Free();
 
             return new BoundAttribute(

src/Compilers/CSharp/Portable/Symbols/Symbol_Attributes.cs

@@ -419,6 +419,10 @@ internal bool LoadAndValidateAttributes(
                         {
                             var boundAttribute = boundAttributeArray[i];
                             Debug.Assert(boundAttribute is not null);
+                            if (boundAttribute.Constructor is { } ctor)
+                            {
+                                Binder.CheckRequiredMembersInObjectInitializer(ctor, ImmutableArray<BoundExpression>.CastUp(boundAttribute.NamedArguments), boundAttribute.Syntax, diagnostics);
+                            }
                             NullableWalker.AnalyzeIfNeeded(binders[i], boundAttribute, boundAttribute.Syntax, diagnostics.DiagnosticBag);
                         }
                     }

src/Compilers/CSharp/Test/Symbol/Symbols/RequiredMembersTests.cs

@@ -7170,4 +7170,33 @@ public class C2 : C1
             Diagnostic(ErrorCode.ERR_RequiredMemberMustBeSet, "C2").WithArguments("C1.P1").WithLocation(1, 9)
         );
     }
+
+    [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/74108")]
+    public void CycleOnConstructorAppliedToSelf()
+    {
+        var source = """
+            namespace System.Diagnostics;
+
+            [System.AttributeUsage(System.AttributeTargets.Class | System.AttributeTargets.Method, AllowMultiple=true)]
+            public sealed class ConditionalAttribute : Attribute
+            {
+                [Conditional("blah")]
+                public ConditionalAttribute(string condition)
+                {
+                    Condition = condition;
+                }
+
+                public string Condition { get;}
+            }
+            """;
+
+        CreateCompilation(source, targetFramework: TargetFramework.NetStandard20).VerifyEmitDiagnostics(
+            // (6,6): warning CS0436: The type 'ConditionalAttribute' in '' conflicts with the imported type 'ConditionalAttribute' in 'netstandard, Version=2.0.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51'. Using the type defined in ''.
+            //     [Conditional("blah")]
+            Diagnostic(ErrorCode.WRN_SameFullNameThisAggAgg, "Conditional").WithArguments("", "System.Diagnostics.ConditionalAttribute", "netstandard, Version=2.0.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51", "System.Diagnostics.ConditionalAttribute").WithLocation(6, 6),
+            // (6,6): error CS0592: Attribute 'Conditional' is not valid on this declaration type. It is only valid on 'class, method' declarations.
+            //     [Conditional("blah")]
+            Diagnostic(ErrorCode.ERR_AttributeOnBadSymbolType, "Conditional").WithArguments("Conditional", "class, method").WithLocation(6, 6)
+        );
+    }
 }