Give 'typeof'-like errors in attribute type arguments

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

@@ -1313,10 +1313,36 @@ private BoundExpression BindTypeOf(TypeOfExpressionSyntax node, BindingDiagnosti
                 hasError = true;
             }
 
+            if (!hasError)
+            {
+                CheckDisallowedAttributeDependentType(typeWithAnnotations, isError: false, node.Location, diagnostics);
+            }
+
             BoundTypeExpression boundType = new BoundTypeExpression(typeSyntax, alias, typeWithAnnotations, type.IsErrorType());
             return new BoundTypeOfOperator(node, boundType, null, this.GetWellKnownType(WellKnownType.System_Type, diagnostics, node), hasError);
         }
 
+        /// <summary>Called when an "attribute-dependent" type such as 'dynamic', 'string?', etc. is not permitted.</summary>
+        private void CheckDisallowedAttributeDependentType(TypeWithAnnotations typeArgument, bool isError, Location errorLocation, BindingDiagnosticBag diagnostics)
+        {
+            var diagnosticId = isError ? ErrorCode.ERR_AttrDependentTypeNotAllowed : ErrorCode.WRN_AttrDependentTypeNotAllowed;
+            typeArgument.VisitType(type: null, static (typeWithAnnotations, arg, _) =>
+            {
+                var (topLevelType, diagnosticId, errorLocation, diagnostics) = arg;
+                var type = typeWithAnnotations.Type;
+                if (type.IsDynamic()
+                    || (typeWithAnnotations.NullableAnnotation.IsAnnotated() && type.IsReferenceType)
+                    || type.IsNativeIntegerType
+                    || (type.IsTupleType && !type.TupleElementNames.IsDefault))
+                {
+                    diagnostics.Add(diagnosticId, errorLocation, topLevelType.ToDisplayString(SymbolDisplayFormat.CSharpShortErrorMessageFormat));
+                    return true;
+                }
+
+                return false;
+            }, typePredicate: null, arg: (typeArgument, diagnosticId, errorLocation, diagnostics));
+        }
+
         private BoundExpression BindSizeOf(SizeOfExpressionSyntax node, BindingDiagnosticBag diagnostics)
         {
             ExpressionSyntax typeSyntax = node.Type;

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

@@ -1175,11 +1175,22 @@ private TypeWithAnnotations BindGenericSimpleNamespaceOrTypeOrAliasSymbol(
             {
                 var boundTypeArguments = BindTypeArguments(typeArguments, diagnostics, basesBeingResolved);
                 if (unconstructedType.IsGenericType
-                    && options.IsAttributeTypeLookup()
-                    && boundTypeArguments.FirstOrDefault(bta => bta.Type.IsUnboundGenericType() || bta.Type.ContainsTypeParameter()) is { HasType: true } badAttributeArgument)
+                    && options.IsAttributeTypeLookup())
                 {
-                    diagnostics.Add(ErrorCode.ERR_AttrTypeArgCannotBeTypeVar, node.Location, badAttributeArgument.ToDisplayString(SymbolDisplayFormat.CSharpErrorMessageFormat));
+                    foreach (var typeArgument in boundTypeArguments)
+                    {
+                        var type = typeArgument.Type;
+                        if (type.IsUnboundGenericType() || type.ContainsTypeParameter())
+                        {
+                            diagnostics.Add(ErrorCode.ERR_AttrTypeArgCannotBeTypeVar, node.Location, type.ToDisplayString(SymbolDisplayFormat.CSharpErrorMessageFormat));
+                        }
+                        else
+                        {
+                            CheckDisallowedAttributeDependentType(typeArgument, isError: true, node.Location, diagnostics);
+                        }
+                    }
                 }
+
                 // It's not an unbound type expression, so we must have type arguments, and we have a
                 // generic type of the correct arity in hand (possibly an error type). Bind the type
                 // arguments and construct the final result.

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -1308,6 +1308,12 @@
   <data name="ERR_AttrTypeArgCannotBeTypeVar" xml:space="preserve">
     <value>'{0}': an attribute type argument cannot use type parameters</value>
   </data>
+  <data name="WRN_AttrDependentTypeNotAllowed" xml:space="preserve">
+    <value>Type '{0}' cannot be used in this context because it cannot be represented in metadata.</value>
+  </data>
+  <data name="ERR_AttrDependentTypeNotAllowed" xml:space="preserve">
+    <value>Type '{0}' cannot be used in this context because it cannot be represented in metadata.</value>
+  </data>
   <data name="ERR_NewTyvarWithArgs" xml:space="preserve">
     <value>'{0}': cannot provide arguments when creating an instance of a variable type</value>
   </data>

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

@@ -1982,6 +1982,8 @@ internal enum ErrorCode
         ERR_FileScopedNamespaceNotBeforeAllMembers = 8956,
         ERR_NoImplicitConvTargetTypedConditional = 8957,
         ERR_AttrTypeArgCannotBeTypeVar = 8958,
+        WRN_AttrDependentTypeNotAllowed = 8959,
+        ERR_AttrDependentTypeNotAllowed = 8960
 
         #endregion
 

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

@@ -205,6 +205,7 @@ internal static int GetWarningLevel(ErrorCode code)
             switch (code)
             {
                 case ErrorCode.WRN_PartialMethodTypeDifference:
+                case ErrorCode.WRN_AttrDependentTypeNotAllowed:
                     // Warning level 6 is exclusively for warnings introduced in the compiler
                     // shipped with dotnet 6 (C# 10) and that can be reported for pre-existing code.
                     return 6;