Error when required members are not set (#60101)

Implements reading the required members list of a type, and enforces that required members are all set by an initializer on the constructor. Required members must be initialized with values, not with nested object initializers.

Test plan #57046.
Specification https://github.com/dotnet/csharplang/blob/main/proposals/required-members.md

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

@@ -194,7 +194,8 @@ private BoundAttribute BindAttributeCore(AttributeSyntax node, NamedTypeSymbol a
                     diagnostics,
                     out var memberResolutionResult,
                     out var candidateConstructors,
-                    allowProtectedConstructorsOfBaseType: true);
+                    allowProtectedConstructorsOfBaseType: true,
+                    suppressUnsupportedRequiredMembersError: false);
                 attributeConstructor = memberResolutionResult.Member;
                 expanded = memberResolutionResult.Resolution == MemberResolutionKind.ApplicableInExpandedForm;
                 argsToParamsOpt = memberResolutionResult.Result.ArgsToParamsOpt;
@@ -238,6 +239,11 @@ private BoundAttribute BindAttributeCore(AttributeSyntax node, NamedTypeSymbol a
             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/Binder/Binder_Expressions.cs

@@ -4064,7 +4064,8 @@ private BoundExpression BindConstructorInitializerCore(
                                     diagnostics,
                                     out memberResolutionResult,
                                     out candidateConstructors,
-                                    allowProtectedConstructorsOfBaseType: true);
+                                    allowProtectedConstructorsOfBaseType: true,
+                                    suppressUnsupportedRequiredMembersError: true);
                 MethodSymbol resultMember = memberResolutionResult.Member;
 
                 validateRecordCopyConstructor(constructor, baseType, resultMember, errorLocation, diagnostics);
@@ -4967,6 +4968,102 @@ private static void ReportDuplicateObjectMemberInitializers(BoundExpression boun
             }
         }
 
+#nullable enable
+        private static void CheckRequiredMembersInObjectInitializer(
+            MethodSymbol constructor,
+            ImmutableArray<BoundExpression> initializers,
+            SyntaxNode creationSyntax,
+            BindingDiagnosticBag diagnostics)
+        {
+            if (!constructor.ShouldCheckRequiredMembers())
+            {
+                return;
+            }
+
+            if (constructor.ContainingType.HasRequiredMembersError)
+            {
+                // An error will be reported on the constructor if from source, or a use-site diagnostic will be reported on the use if from metadata.
+                return;
+            }
+
+            var requiredMembers = constructor.ContainingType.AllRequiredMembers;
+
+            if (requiredMembers.Count == 0)
+            {
+                return;
+            }
+
+            var requiredMembersBuilder = requiredMembers.ToBuilder();
+
+            if (initializers.IsDefaultOrEmpty)
+            {
+                reportMembers();
+                return;
+            }
+
+            foreach (var initializer in initializers)
+            {
+                if (initializer is not BoundAssignmentOperator assignmentOperator)
+                {
+                    continue;
+                }
+
+                var memberSymbol = assignmentOperator.Left switch
+                {
+                    // Regular initializers
+                    BoundObjectInitializerMember member => member.MemberSymbol,
+                    // Attribute initializers
+                    BoundPropertyAccess propertyAccess => propertyAccess.PropertySymbol,
+                    BoundFieldAccess fieldAccess => fieldAccess.FieldSymbol,
+                    // Error cases
+                    _ => null
+                };
+
+                if (memberSymbol is null)
+                {
+                    continue;
+                }
+
+                if (!requiredMembersBuilder.TryGetValue(memberSymbol.Name, out var requiredMember))
+                {
+                    continue;
+                }
+
+                if (!memberSymbol.Equals(requiredMember, TypeCompareKind.ConsiderEverything))
+                {
+                    continue;
+                }
+
+                requiredMembersBuilder.Remove(memberSymbol.Name);
+
+                if (assignmentOperator.Right is BoundObjectInitializerExpressionBase initializerExpression)
+                {
+                    // Required member '{0}' must be assigned a value, it cannot use a nested member or collection initializer.
+                    diagnostics.Add(ErrorCode.ERR_RequiredMembersMustBeAssignedValue, initializerExpression.Syntax.Location, requiredMember);
+                }
+            }
+
+            reportMembers();
+
+            void reportMembers()
+            {
+                Location location = creationSyntax switch
+                {
+                    ObjectCreationExpressionSyntax { Type: { } type } => type.Location,
+                    BaseObjectCreationExpressionSyntax { NewKeyword: { } newKeyword } => newKeyword.GetLocation(),
+                    AttributeSyntax { Name: { } name } => name.Location,
+                    _ => creationSyntax.Location
+                };
+
+                foreach (var (_, member) in requiredMembersBuilder)
+                {
+                    // Required member '{0}' must be set in the object initializer or attribute constructor.
+                    diagnostics.Add(ErrorCode.ERR_RequiredMemberMustBeSet, location, member);
+                }
+            }
+        }
+#nullable disable
+
         private BoundCollectionInitializerExpression BindCollectionInitializerExpression(
             InitializerExpressionSyntax initializerSyntax,
             TypeSymbol initializerType,
@@ -5365,7 +5462,8 @@ protected BoundExpression BindClassCreationExpression(
                     diagnostics,
                     out MemberResolutionResult<MethodSymbol> memberResolutionResult,
                     out ImmutableArray<MethodSymbol> candidateConstructors,
-                    allowProtectedConstructorsOfBaseType: false) &&
+                    allowProtectedConstructorsOfBaseType: false,
+                    suppressUnsupportedRequiredMembersError: false) &&
                 !type.IsAbstract)
             {
                 var method = memberResolutionResult.Member;
@@ -5411,7 +5509,7 @@ protected BoundExpression BindClassCreationExpression(
                 }
 
                 boundInitializerOpt = makeBoundInitializerOpt();
-                result = new BoundObjectCreationExpression(
+                var creation = new BoundObjectCreationExpression(
                     node,
                     method,
                     candidateConstructors,
@@ -5427,7 +5525,9 @@ protected BoundExpression BindClassCreationExpression(
                     type,
                     hasError);
 
-                return result;
+                CheckRequiredMembersInObjectInitializer(creation.Constructor, creation.InitializerExpressionOpt?.Initializers ?? default, creation.Syntax, diagnostics);
+
+                return creation;
             }
 
             LookupResultKind resultKind;
@@ -5699,7 +5799,8 @@ internal bool TryPerformConstructorOverloadResolution(
             BindingDiagnosticBag diagnostics,
             out MemberResolutionResult<MethodSymbol> memberResolutionResult,
             out ImmutableArray<MethodSymbol> candidateConstructors,
-            bool allowProtectedConstructorsOfBaseType) // Last to make named arguments more convenient.
+            bool allowProtectedConstructorsOfBaseType,
+            bool suppressUnsupportedRequiredMembersError) // Last to make named arguments more convenient.
         {
             // Get accessible constructors for performing overload resolution.
             ImmutableArray<MethodSymbol> allInstanceConstructors;
@@ -5747,7 +5848,25 @@ internal bool TryPerformConstructorOverloadResolution(
                 }
             }
 
-            diagnostics.Add(errorLocation, useSiteInfo);
+            if (suppressUnsupportedRequiredMembersError && useSiteInfo.AccumulatesDiagnostics && useSiteInfo.Diagnostics is { Count: not 0 })
+            {
+                diagnostics.AddDependencies(useSiteInfo);
+                foreach (var diagnostic in useSiteInfo.Diagnostics)
+                {
+                    // We don't want to report this error here because we'll report ERR_RequiredMembersBaseTypeInvalid. That error is suppressable by the
+                    // user using the `SetsRequiredMembers` attribute on the constructor, so reporting this error would prevent that from working.
+                    if ((ErrorCode)diagnostic.Code == ErrorCode.ERR_RequiredMembersInvalid)
+                    {
+                        continue;
+                    }
+
+                    diagnostics.ReportUseSiteDiagnostic(diagnostic, errorLocation);
+                }
+            }
+            else
+            {
+                diagnostics.Add(errorLocation, useSiteInfo);
+            }
 
             if (succeededIgnoringAccessibility)
             {

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -7007,6 +7007,18 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
   <data name="ERR_RequiredMemberMustBeSettable" xml:space="preserve">
     <value>Required member '{0}' must be settable.</value>
   </data>
+  <data name="ERR_RequiredMemberMustBeSet" xml:space="preserve">
+    <value>Required member '{0}' must be set in the object initializer or attribute constructor.</value>
+  </data>
+  <data name="ERR_RequiredMembersMustBeAssignedValue" xml:space="preserve">
+    <value>Required member '{0}' must be assigned a value, it cannot use a nested member or collection initializer.</value>
+  </data>
+  <data name="ERR_RequiredMembersInvalid" xml:space="preserve">
+    <value>The required members list for '{0}' is malformed and cannot be interpreted.</value>
+  </data>
+  <data name="ERR_RequiredMembersBaseTypeInvalid" xml:space="preserve">
+    <value>The required members list for the base type '{0}' is malformed and cannot be interpreted. To use this constructor, apply the 'SetsRequiredMembers' attribute.</value>
+  </data>
   <data name="ERR_LineContainsDifferentWhitespace" xml:space="preserve">
     <value>Line contains different whitespace than the closing line of the raw string literal: '{0}' versus '{1}'</value>
   </data>

src/Compilers/CSharp/Portable/Errors/ErrorCode.cs

@@ -2062,5 +2062,9 @@ internal enum ErrorCode
         ERR_RequiredMemberCannotBeLessVisibleThanContainingType = 9503,
         ERR_ExplicitRequiredMember = 9504,
         ERR_RequiredMemberMustBeSettable = 9505,
+        ERR_RequiredMemberMustBeSet = 9506,
+        ERR_RequiredMembersMustBeAssignedValue = 9507,
+        ERR_RequiredMembersInvalid = 9508,
+        ERR_RequiredMembersBaseTypeInvalid = 9509,
     }
 }

src/Compilers/CSharp/Portable/Symbols/Metadata/PE/PEMethodSymbol.cs

@@ -1346,6 +1346,11 @@ internal override UseSiteInfo<AssemblySymbol> GetUseSiteInfo()
                     }
                 }
 
+                if (diagnosticInfo == null && this.ShouldCheckRequiredMembers() && ContainingType.HasRequiredMembersError)
+                {
+                    diagnosticInfo = new CSDiagnosticInfo(ErrorCode.ERR_RequiredMembersInvalid, ContainingType);
+                }
+
                 return InitializeUseSiteDiagnostic(result.AdjustDiagnosticInfo(diagnosticInfo));
             }
 

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

@@ -11,6 +11,8 @@
 using System.Linq;
 using System.Runtime.CompilerServices;
 using System.Runtime.InteropServices;
+using System.Threading;
+using Microsoft.CodeAnalysis.Collections;
 using Microsoft.CodeAnalysis.PooledObjects;
 using Microsoft.CodeAnalysis.Symbols;
 using Roslyn.Utilities;
@@ -24,6 +26,14 @@ internal abstract partial class NamedTypeSymbol : TypeSymbol, INamedTypeSymbolIn
     {
         private bool _hasNoBaseCycles;
 
+        private static readonly ImmutableSegmentedDictionary<string, Symbol> RequiredMembersErrorSentinel = ImmutableSegmentedDictionary<string, Symbol>.Empty.Add("<error sentinel>", null!);
+
+        /// <summary>
+        /// <see langword="default"/> if uninitialized. <see cref="RequiredMembersErrorSentinel"/> if there are errors. <see cref="ImmutableSegmentedDictionary{TKey, TValue}.Empty"/> if
+        /// there are no required members. Otherwise, the required members.
+        /// </summary>
+        private ImmutableSegmentedDictionary<string, Symbol> _lazyRequiredMembers = default;
+
         // Only the compiler can create NamedTypeSymbols.
         internal NamedTypeSymbol(TupleExtraData tupleData = null)
         {
@@ -499,6 +509,122 @@ internal abstract bool MangleName
         /// </summary>
         internal abstract bool HasDeclaredRequiredMembers { get; }
 
+#nullable enable
+        /// <summary>
+        /// Whether the type encountered an error while trying to build its complete list of required members.
+        /// </summary>
+        internal bool HasRequiredMembersError
+        {
+            get
+            {
+                CalculateRequiredMembersIfRequired();
+                Debug.Assert(!_lazyRequiredMembers.IsDefault);
+                return _lazyRequiredMembers == RequiredMembersErrorSentinel;
+            }
+        }
+
+        /// <summary>
+        /// The full list of all required members for this type, including from base classes. If <see cref="HasRequiredMembersError"/> is true,
+        /// this returns empty.
+        /// </summary>
+        /// <remarks>
+        /// Do not call this API if all you need are the required members declared on this type. Use <see cref="GetMembers()"/> instead, filtering for
+        /// required members, instead of calling this API.
+        /// </remarks>
+        internal ImmutableSegmentedDictionary<string, Symbol> AllRequiredMembers
+        {
+            get
+            {
+                CalculateRequiredMembersIfRequired();
+                Debug.Assert(!_lazyRequiredMembers.IsDefault);
+                if (_lazyRequiredMembers == RequiredMembersErrorSentinel)
+                {
+                    return ImmutableSegmentedDictionary<string, Symbol>.Empty;
+                }
+
+                return _lazyRequiredMembers;
+            }
+        }
+
+        private void CalculateRequiredMembersIfRequired()
+        {
+            if (!_lazyRequiredMembers.IsDefault)
+            {
+                return;
+            }
+
+            ImmutableSegmentedDictionary<string, Symbol>.Builder? builder = null;
+            bool success = TryCalculateRequiredMembers(ref builder);
+
+            var requiredMembers = success
+                ? builder?.ToImmutable() ?? ImmutableSegmentedDictionary<string, Symbol>.Empty
+                : RequiredMembersErrorSentinel;
+
+            RoslynImmutableInterlocked.InterlockedInitialize(ref _lazyRequiredMembers, requiredMembers);
+        }
+
+        /// <summary>
+        /// Attempts to calculate the required members for this type. Returns false if there were errors.
+        /// </summary>
+        private bool TryCalculateRequiredMembers(ref ImmutableSegmentedDictionary<string, Symbol>.Builder? requiredMembersBuilder)
+        {
+            var lazyRequiredMembers = _lazyRequiredMembers;
+            if (_lazyRequiredMembers == RequiredMembersErrorSentinel)
+            {
+                if (lazyRequiredMembers.IsDefault)
+                {
+                    return false;
+                }
+                else
+                {
+                    requiredMembersBuilder = lazyRequiredMembers.ToBuilder();
+                    return true;
+                }
+            }
+
+            if (BaseTypeNoUseSiteDiagnostics?.TryCalculateRequiredMembers(ref requiredMembersBuilder) == false)
+            {
+                return false;
+            }
+
+            // We need to make sure that members from a base type weren't hidden by members from the current type.
+            if (!HasDeclaredRequiredMembers && requiredMembersBuilder == null)
+            {
+                return true;
+            }
+
+            return addCurrentTypeMembers(ref requiredMembersBuilder);
+
+            bool addCurrentTypeMembers(ref ImmutableSegmentedDictionary<string, Symbol>.Builder? requiredMembersBuilder)
+            {
+                requiredMembersBuilder ??= ImmutableSegmentedDictionary.CreateBuilder<string, Symbol>();
+
+                foreach (var member in GetMembersUnordered())
+                {
+                    if (requiredMembersBuilder.ContainsKey(member.Name))
+                    {
+                        // This is only permitted if the member is an override of a required member from a base type, and is required itself.
+                        if (!member.IsRequired()
+                            || member.GetOverriddenMember() is not { } overriddenMember
+                            || !overriddenMember.Equals(requiredMembersBuilder[member.Name], TypeCompareKind.ConsiderEverything))
+                        {
+                            return false;
+                        }
+                    }
+
+                    if (!member.IsRequired())
+                    {
+                        continue;
+                    }
+
+                    requiredMembersBuilder[member.Name] = member;
+                }
+
+                return true;
+            }
+        }
+#nullable disable
+
         /// <summary>
         /// Get all the members of this symbol.
         /// </summary>