Don't identify dictionary indexer as NRT

Fixes #20718

src/EFCore/Metadata/Conventions/NonNullableConventionBase.cs

@@ -69,9 +69,9 @@ protected virtual bool IsNonNullableReferenceType(
                 return false;
             }
 
-            // For C# 8.0 nullable types, the C# currently synthesizes a NullableAttribute that expresses nullability into assemblies
-            // it produces. If the model is spread across more than one assembly, there will be multiple versions of this attribute,
-            // so look for it by name, caching to avoid reflection on every check.
+            // For C# 8.0 nullable types, the C# compiler currently synthesizes a NullableAttribute that expresses nullability into
+            // assemblies it produces. If the model is spread across more than one assembly, there will be multiple versions of this
+            // attribute, so look for it by name, caching to avoid reflection on every check.
             // Note that this may change - if https://github.com/dotnet/corefx/issues/36222 is done we can remove all of this.
 
             // First look for NullableAttribute on the member itself
@@ -114,6 +114,18 @@ protected virtual bool IsNonNullableReferenceType(
 
                     if (state.NullableContextFlagFieldInfo?.GetValue(contextAttr) is byte flag)
                     {
+                        // We currently don't calculate support nullability for generic properties, since calculating that is complex
+                        // (depends on the nullability of generic type argument).
+                        // However, we special case Dictionary as it's used for property bags, and specifically don't identify its indexer
+                        // as non-nullable.
+                        if (memberInfo is PropertyInfo property
+                            && property.IsIndexerProperty()
+                            && type.IsGenericType
+                            && type.GetGenericTypeDefinition() == typeof(Dictionary<,>))
+                        {
+                            return false;
+                        }
+
                         return state.TypeCache[type] = flag == 1;
                     }
                 }

src/EFCore/Properties/CoreStrings.resx

@@ -1281,7 +1281,7 @@
     <value>Cannot set backing field '{field}' for the indexer property '{entityType}.{property}'. Indexer properties are not allowed to use a backing field.</value>
   </data>
   <data name="ClashingSharedType" xml:space="preserve">
-    <value>The entity type '{entityType}' cannot be added to the model because a shared entity type with the same clr type already exists.</value>
+    <value>The entity type '{entityType}' cannot be added to the model because a shared entity type with the same CLR type already exists.</value>
   </data>
   <data name="SkipNavigationInUseBySkipNavigation" xml:space="preserve">
     <value>The skip navigation '{skipNavigation}' cannot be removed because it is set as the inverse of the skip navigation '{inverseSkipNavigation}' on '{referencingEntityType}'. All referencing skip navigations must be removed before this skip navigation can be removed.</value>

test/EFCore.Specification.Tests/Query/NorthwindKeylessEntitiesQueryTestBase.cs

@@ -161,7 +161,7 @@ where ov.Customer.Orders.Any()
 
         [ConditionalTheory]
         [MemberData(nameof(IsAsyncData))]
-        public virtual Task KeylesEntity_groupby(bool async)
+        public virtual Task KeylessEntity_groupby(bool async)
         {
             return AssertQuery(
                 async,

test/EFCore.SqlServer.FunctionalTests/Query/NorthwindKeylessEntitiesQuerySqlServerTest.cs

@@ -147,9 +147,9 @@ public override void Auto_initialized_view_set()
                 @"SELECT [c].[CustomerID], [c].[Address], [c].[City], [c].[CompanyName], [c].[ContactName], [c].[ContactTitle], [c].[Country], [c].[Fax], [c].[Phone], [c].[PostalCode], [c].[Region] FROM [Customers] AS [c]");
         }
 
-        public override async Task KeylesEntity_groupby(bool async)
+        public override async Task KeylessEntity_groupby(bool async)
         {
-            await base.KeylesEntity_groupby(async);
+            await base.KeylessEntity_groupby(async);
 
             AssertSql(
                 @"SELECT [c].[City] AS [Key], COUNT(*) AS [Count], COALESCE(SUM(CAST(LEN([c].[Address]) AS int)), 0) AS [Sum]

test/EFCore.Tests/Metadata/Conventions/NonNullableReferencePropertyConventionTest.cs

@@ -2,6 +2,7 @@
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
 using System;
+using System.Collections.Generic;
 using System.ComponentModel.DataAnnotations;
 using System.Diagnostics.CodeAnalysis;
 using Microsoft.EntityFrameworkCore.Metadata.Builders;
@@ -82,6 +83,15 @@ public void Reference_nullability_sets_is_nullable_correctly(Type type, string p
             Assert.Equal(expectedNullable, entityTypeBuilder.Property(propertyName).Metadata.IsNullable);
         }
 
+        [ConditionalFact]
+        public void Non_nullability_ignores_context_on_generic_types()
+        {
+            var modelBuilder = CreateModelBuilder();
+            var entityTypeBuilder = modelBuilder.SharedTypeEntity<Dictionary<string, object>>("SomeBag");
+            entityTypeBuilder.IndexerProperty(typeof(string), "b");
+            Assert.True(entityTypeBuilder.Property("b").Metadata.IsNullable);
+        }
+
         private void RunConvention(InternalPropertyBuilder propertyBuilder)
         {
             var context = new ConventionContext<IConventionPropertyBuilder>(