Support nullable annotations on unconstrained type parameters

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

@@ -139,7 +139,7 @@ internal ImmutableArray<TypeParameterConstraintClause> BindTypeParameterConstrai
                         {
                             if (!reportedOverrideWithConstraints)
                             {
-                                diagnostics.Add(ErrorCode.ERR_TypeConstraintsMustBeUniqueAndFirst, syntax.GetFirstToken().GetLocation());
+                                reportTypeConstraintsMustBeUniqueAndFirst(syntax, diagnostics);
                             }
 
                             if (isForOverride && (constraints & (TypeParameterConstraintKind.ValueType | TypeParameterConstraintKind.ReferenceType)) != 0)
@@ -180,7 +180,7 @@ internal ImmutableArray<TypeParameterConstraintClause> BindTypeParameterConstrai
                         {
                             if (!reportedOverrideWithConstraints)
                             {
-                                diagnostics.Add(ErrorCode.ERR_TypeConstraintsMustBeUniqueAndFirst, syntax.GetFirstToken().GetLocation());
+                                reportTypeConstraintsMustBeUniqueAndFirst(syntax, diagnostics);
                             }
 
                             if (isForOverride && (constraints & (TypeParameterConstraintKind.ValueType | TypeParameterConstraintKind.ReferenceType)) != 0)
@@ -214,6 +214,29 @@ internal ImmutableArray<TypeParameterConstraintClause> BindTypeParameterConstrai
 
                         constraints |= TypeParameterConstraintKind.Constructor;
                         continue;
+                    case SyntaxKind.DefaultConstraint:
+                        CheckFeatureAvailability(syntax, MessageID.IDS_FeatureDefaultTypeParameterConstraint, diagnostics);
+
+                        if (!isForOverride)
+                        {
+                            diagnostics.Add(ErrorCode.ERR_DefaultConstraintOverrideOnly, syntax.GetLocation());
+                        }
+
+                        if (i != 0)
+                        {
+                            if (!reportedOverrideWithConstraints)
+                            {
+                                reportTypeConstraintsMustBeUniqueAndFirst(syntax, diagnostics);
+                            }
+
+                            if (isForOverride && (constraints & (TypeParameterConstraintKind.ValueType | TypeParameterConstraintKind.ReferenceType)) != 0)
+                            {
+                                continue;
+                            }
+                        }
+
+                        constraints |= TypeParameterConstraintKind.Default;
+                        continue;
                     case SyntaxKind.TypeConstraint:
                         if (isForOverride)
                         {
@@ -239,7 +262,7 @@ internal ImmutableArray<TypeParameterConstraintClause> BindTypeParameterConstrai
                                 case ConstraintContextualKeyword.Unmanaged:
                                     if (i != 0)
                                     {
-                                        diagnostics.Add(ErrorCode.ERR_TypeConstraintsMustBeUniqueAndFirst, typeSyntax.GetLocation());
+                                        reportTypeConstraintsMustBeUniqueAndFirst(typeSyntax, diagnostics);
                                         continue;
                                     }
 
@@ -253,7 +276,7 @@ internal ImmutableArray<TypeParameterConstraintClause> BindTypeParameterConstrai
                                 case ConstraintContextualKeyword.NotNull:
                                     if (i != 0)
                                     {
-                                        diagnostics.Add(ErrorCode.ERR_TypeConstraintsMustBeUniqueAndFirst, typeSyntax.GetLocation());
+                                        reportTypeConstraintsMustBeUniqueAndFirst(typeSyntax, diagnostics);
                                     }
 
                                     constraints |= TypeParameterConstraintKind.NotNull;
@@ -292,6 +315,11 @@ static void reportOverrideWithConstraints(ref bool reportedOverrideWithConstrain
                     reportedOverrideWithConstraints = true;
                 }
             }
+
+            static void reportTypeConstraintsMustBeUniqueAndFirst(CSharpSyntaxNode syntax, DiagnosticBag diagnostics)
+            {
+                diagnostics.Add(ErrorCode.ERR_TypeConstraintsMustBeUniqueAndFirst, syntax.GetLocation());
+            }
         }
 
         internal ImmutableArray<TypeParameterConstraintClause> GetDefaultTypeParameterConstraintClauses(TypeParameterListSyntax typeParameterList)
@@ -351,7 +379,7 @@ private static TypeParameterConstraintClause RemoveInvalidConstraints(
                     // since, in general, it may be difficult to support all invalid types.
                     // In the future, we may want to include some invalid types
                     // though so the public binding API has the most information.
-                    if (Binder.IsValidConstraint(typeParameter.Name, syntax, constraintType, constraintClause.Constraints, constraintTypeBuilder, diagnostics))
+                    if (IsValidConstraint(typeParameter.Name, syntax, constraintType, constraintClause.Constraints, constraintTypeBuilder, diagnostics))
                     {
                         CheckConstraintTypeVisibility(containingSymbol, syntax.Location, constraintType, diagnostics);
                         constraintTypeBuilder.Add(constraintType);
@@ -388,7 +416,7 @@ private static void CheckConstraintTypeVisibility(
         /// Returns true if the constraint is valid. Otherwise
         /// returns false and generates a diagnostic.
         /// </summary>
-        internal static bool IsValidConstraint(
+        private static bool IsValidConstraint(
             string typeParameterName,
             TypeConstraintSyntax syntax,
             TypeWithAnnotations type,

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

@@ -1020,7 +1020,7 @@ private BoundPattern BindVarDesignation(
                     }
                 case SyntaxKind.SingleVariableDesignation:
                     {
-                        var declType = TypeWithState.ForType(inputType).ToTypeWithAnnotations();
+                        var declType = TypeWithState.ForType(inputType).ToTypeWithAnnotations(Compilation);
                         BindPatternDesignation(
                             designation: node, declType: declType, inputValEscape: inputValEscape, permitDesignations: permitDesignations,
                             typeSyntax: null, diagnostics: diagnostics, hasErrors: ref hasErrors,

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

@@ -512,7 +512,7 @@ NamespaceOrTypeOrAliasSymbolWithAnnotations bindNullable()
 
                 if (!ShouldCheckConstraints)
                 {
-                    diagnostics.Add(new LazyUseSiteDiagnosticsInfoForNullableType(constructedType), syntax.GetLocation());
+                    diagnostics.Add(new LazyUseSiteDiagnosticsInfoForNullableType(Compilation.LanguageVersion, constructedType), syntax.GetLocation());
                 }
                 else if (constructedType.IsNullableType())
                 {
@@ -521,9 +521,9 @@ NamespaceOrTypeOrAliasSymbolWithAnnotations bindNullable()
                     var location = syntax.Location;
                     type.CheckConstraints(new ConstraintsHelper.CheckConstraintsArgs(this.Compilation, this.Conversions, includeNullability: true, location, diagnostics));
                 }
-                else if (constructedType.Type.IsTypeParameterDisallowingAnnotation())
+                else if (GetNullableUnconstrainedTypeParameterDiagnosticIfNecessary(Compilation.LanguageVersion, constructedType) is { } diagnosticInfo)
                 {
-                    diagnostics.Add(ErrorCode.ERR_NullableUnconstrainedTypeParameter, syntax.Location);
+                    diagnostics.Add(diagnosticInfo, syntax.Location);
                 }
 
                 return constructedType;
@@ -571,6 +571,19 @@ NamespaceOrTypeOrAliasSymbolWithAnnotations createErrorType()
                 return TypeWithAnnotations.Create(CreateErrorType());
             }
         }
+
+        internal static CSDiagnosticInfo? GetNullableUnconstrainedTypeParameterDiagnosticIfNecessary(LanguageVersion languageVersion, in TypeWithAnnotations type)
+        {
+            if (type.Type.IsTypeParameterDisallowingAnnotation())
+            {
+                var requiredVersion = MessageID.IDS_FeatureDefaultTypeParameterConstraint.RequiredVersion();
+                if (requiredVersion > languageVersion)
+                {
+                    return new CSDiagnosticInfo(ErrorCode.ERR_NullableUnconstrainedTypeParameter, new CSharpRequiredLanguageVersion(requiredVersion));
+                }
+            }
+            return null;
+        }
 #nullable restore
 
         private TypeWithAnnotations BindArrayType(

src/Compilers/CSharp/Portable/Binder/Semantics/Conversions/ConversionsBase.cs

@@ -1493,12 +1493,14 @@ internal bool HasTopLevelNullabilityIdentityConversion(TypeWithAnnotations sourc
                 return true;
             }
 
-            if (source.IsPossiblyNullableTypeTypeParameter() && !destination.IsPossiblyNullableTypeTypeParameter())
+            var sourceIsPossiblyNullableTypeParameter = IsPossiblyNullableTypeTypeParameter(source);
+            var destinationIsPossiblyNullableTypeParameter = IsPossiblyNullableTypeTypeParameter(destination);
+            if (sourceIsPossiblyNullableTypeParameter && !destinationIsPossiblyNullableTypeParameter)
             {
                 return destination.NullableAnnotation.IsAnnotated();
             }
 
-            if (destination.IsPossiblyNullableTypeTypeParameter() && !source.IsPossiblyNullableTypeTypeParameter())
+            if (destinationIsPossiblyNullableTypeParameter && !sourceIsPossiblyNullableTypeParameter)
             {
                 return source.NullableAnnotation.IsAnnotated();
             }
@@ -1523,14 +1525,21 @@ internal bool HasTopLevelNullabilityImplicitConversion(TypeWithAnnotations sourc
                 return true;
             }
 
-            if (source.IsPossiblyNullableTypeTypeParameter() && !destination.IsPossiblyNullableTypeTypeParameter())
+            if (IsPossiblyNullableTypeTypeParameter(source) && !IsPossiblyNullableTypeTypeParameter(destination))
             {
                 return false;
             }
 
             return !source.NullableAnnotation.IsAnnotated();
         }
 
+        private static bool IsPossiblyNullableTypeTypeParameter(in TypeWithAnnotations typeWithAnnotations)
+        {
+            var type = typeWithAnnotations.Type;
+            return type is object &&
+                (type.IsPossiblyNullableReferenceTypeTypeParameter() || type.IsNullableTypeOrTypeParameter());
+        }
+
         /// <summary>
         /// Returns false if the source does not have an implicit conversion to the destination
         /// because of either incompatible top level or nested nullability.

src/Compilers/CSharp/Portable/Binder/Semantics/OverloadResolution/MethodTypeInference.cs

@@ -1541,9 +1541,9 @@ private bool ExactOrBoundsNullableInference(ExactOrBoundsKind kind, TypeWithAnno
 
             return false;
 
-            // True if the type is nullable but not an unconstrained type parameter.
+            // True if the type is nullable.
             bool isNullableOnly(TypeWithAnnotations type)
-                => type.NullableAnnotation.IsAnnotated() && !type.Type.IsTypeParameterDisallowingAnnotation();
+                => type.NullableAnnotation.IsAnnotated();
         }
 
         private bool ExactNullableInference(TypeWithAnnotations source, TypeWithAnnotations target, ref HashSet<DiagnosticInfo> useSiteDiagnostics)

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -1363,7 +1363,7 @@
     <value>The return type for ++ or -- operator must match the parameter type or be derived from the parameter type</value>
   </data>
   <data name="ERR_TypeConstraintsMustBeUniqueAndFirst" xml:space="preserve">
-    <value>The 'class', 'struct', 'unmanaged', and 'notnull' constraints cannot be combined or duplicated, and must be specified first in the constraints list.</value>
+    <value>The 'class', 'struct', 'unmanaged', 'notnull', and 'default' constraints cannot be combined or duplicated, and must be specified first in the constraints list.</value>
   </data>
   <data name="ERR_RefValBoundWithClass" xml:space="preserve">
     <value>'{0}': cannot specify both a constraint class and the 'class' or 'struct' constraint</value>
@@ -5603,7 +5603,7 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
     <value>Explicit application of 'System.Runtime.CompilerServices.NullableAttribute' is not allowed.</value>
   </data>
   <data name="ERR_NullableUnconstrainedTypeParameter" xml:space="preserve">
-    <value>A nullable type parameter must be known to be a value type or non-nullable reference type. Consider adding a 'class', 'struct', or type constraint.</value>
+    <value>A nullable type parameter must be known to be a value type or non-nullable reference type. Please use language version '{0}' or greater, or consider adding a 'class', 'struct', or type constraint.</value>
   </data>
   <data name="ERR_NullableOptionNotAvailable" xml:space="preserve">
     <value>Invalid '{0}' value: '{1}' for C# {2}. Please use language version '{3}' or greater.</value>
@@ -5731,6 +5731,9 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
   <data name="IDS_FeatureNullPointerConstantPattern" xml:space="preserve">
     <value>null pointer constant pattern</value>
   </data>
+  <data name="IDS_FeatureDefaultTypeParameterConstraint" xml:space="preserve">
+    <value>default type parameter constraints</value>
+  </data>
   <data name="ERR_WrongNumberOfSubpatterns" xml:space="preserve">
     <value>Matching the tuple type '{0}' requires '{1}' subpatterns, but '{2}' subpatterns are present.</value>
   </data>
@@ -6016,6 +6019,12 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
   <data name="ERR_OverrideValConstraintNotSatisfied" xml:space="preserve">
     <value>Method '{0}' specifies a 'struct' constraint for type parameter '{1}', but corresponding type parameter '{2}' of overridden or explicitly implemented method '{3}' is not a non-nullable value type.</value>
   </data>
+  <data name="ERR_OverrideDefaultConstraintNotSatisfied" xml:space="preserve">
+    <value>Method '{0}' specifies a 'default' constraint for type parameter '{1}', but corresponding type parameter '{2}' of overridden or explicitly implemented method '{3}' is constrained to a reference type or a value type.</value>
+  </data>
+  <data name="ERR_DefaultConstraintOverrideOnly" xml:space="preserve">
+    <value>The 'default' constraint is valid on override and explicit interface implementation methods only.</value>
+  </data>
   <data name="IDS_OverrideWithConstraints" xml:space="preserve">
     <value>constraints for override and explicit interface implementation methods</value>
   </data>

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

@@ -1834,6 +1834,8 @@ internal enum ErrorCode
 
         ERR_StaticAnonymousFunctionCannotCaptureVariable = 8820,
         ERR_StaticAnonymousFunctionCannotCaptureThis = 8821,
+        ERR_OverrideDefaultConstraintNotSatisfied = 8822,
+        ERR_DefaultConstraintOverrideOnly = 8823,
 
         ERR_ExpressionTreeContainsWithExpression = 8849,
         ERR_BadRecordDeclaration = 8850,

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

@@ -2,13 +2,15 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using System.Threading;
 
 namespace Microsoft.CodeAnalysis.CSharp
 {
     internal abstract class LazyDiagnosticInfo : DiagnosticInfo
     {
-        private DiagnosticInfo _lazyInfo;
+        private DiagnosticInfo? _lazyInfo;
 
         protected LazyDiagnosticInfo()
             : base(CSharp.MessageProvider.Instance, (int)ErrorCode.Unknown)
@@ -25,6 +27,6 @@ internal sealed override DiagnosticInfo GetResolvedInfo()
             return _lazyInfo;
         }
 
-        protected abstract DiagnosticInfo ResolveInfo();
+        protected abstract DiagnosticInfo? ResolveInfo();
     }
 }

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

@@ -2,31 +2,30 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using Microsoft.CodeAnalysis.CSharp.Symbols;
 
 namespace Microsoft.CodeAnalysis.CSharp
 {
     internal sealed class LazyUseSiteDiagnosticsInfoForNullableType : LazyDiagnosticInfo
     {
+        private readonly LanguageVersion _languageVersion;
         private readonly TypeWithAnnotations _possiblyNullableTypeSymbol;
 
-        internal LazyUseSiteDiagnosticsInfoForNullableType(TypeWithAnnotations possiblyNullableTypeSymbol)
+        internal LazyUseSiteDiagnosticsInfoForNullableType(LanguageVersion languageVersion, TypeWithAnnotations possiblyNullableTypeSymbol)
         {
+            _languageVersion = languageVersion;
             _possiblyNullableTypeSymbol = possiblyNullableTypeSymbol;
         }
 
-        protected override DiagnosticInfo ResolveInfo()
+        protected override DiagnosticInfo? ResolveInfo()
         {
             if (_possiblyNullableTypeSymbol.IsNullableType())
             {
                 return _possiblyNullableTypeSymbol.Type.OriginalDefinition.GetUseSiteDiagnostic();
             }
-            else if (_possiblyNullableTypeSymbol.Type.IsTypeParameterDisallowingAnnotation())
-            {
-                return new CSDiagnosticInfo(ErrorCode.ERR_NullableUnconstrainedTypeParameter);
-            }
-
-            return null;
+            return Binder.GetNullableUnconstrainedTypeParameterDiagnosticIfNecessary(_languageVersion, _possiblyNullableTypeSymbol);
         }
     }
 }

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

@@ -207,6 +207,7 @@ internal enum MessageID
         IDS_FeatureInitOnlySetters = MessageBase + 12781,
         IDS_FeatureRecords = MessageBase + 12782,
         IDS_FeatureNullPointerConstantPattern = MessageBase + 12783,
+        IDS_FeatureDefaultTypeParameterConstraint = MessageBase + 12784,
     }
 
     // Message IDs may refer to strings that need to be localized.
@@ -332,6 +333,7 @@ internal static LanguageVersion RequiredVersion(this MessageID feature)
                 case MessageID.IDS_FeatureInitOnlySetters: // semantic check
                 case MessageID.IDS_FeatureRecords:
                 case MessageID.IDS_FeatureStaticAnonymousFunction: // syntax check
+                case MessageID.IDS_FeatureDefaultTypeParameterConstraint:
                     return LanguageVersion.Preview;
 
                 // C# 8.0 features.