Improve exception message for a shadow skip navigation

src/EFCore/Infrastructure/ModelValidator.cs

@@ -130,6 +130,16 @@ protected virtual void ValidateRelationships(
                             CoreStrings.SkipNavigationNoInverse(
                                 skipNavigation.Name, skipNavigation.DeclaringEntityType.DisplayName()));
                     }
+
+                    if (skipNavigation.IsShadowProperty())
+                    {
+                        throw new InvalidOperationException(
+                            CoreStrings.ShadowManyToManyNavigation(
+                                skipNavigation.DeclaringEntityType.DisplayName(),
+                                skipNavigation.Name,
+                                skipNavigation.Inverse.DeclaringEntityType.DisplayName(),
+                                skipNavigation.Inverse.Name));
+                    }
                 }
             }
         }

src/EFCore/Metadata/Builders/CollectionNavigationBuilder`.cs

@@ -95,6 +95,15 @@ public virtual ReferenceCollectionBuilder<TEntity, TRelatedEntity> WithOne(
         /// <returns> An object to further configure the relationship. </returns>
         public new virtual CollectionCollectionBuilder<TRelatedEntity, TEntity> WithMany(string navigationName)
         {
+            if (Builder != null
+                && Builder.Metadata.PrincipalToDependent == null)
+            {
+                throw new InvalidOperationException(
+                    CoreStrings.MissingInverseManyToManyNavigation(
+                        Builder.Metadata.PrincipalEntityType.DisplayName(),
+                        Builder.Metadata.DeclaringEntityType.DisplayName()));
+            }
+
             var leftName = Builder?.Metadata.PrincipalToDependent!.Name;
             var collectionCollectionBuilder =
                 new CollectionCollectionBuilder<TRelatedEntity, TEntity>(

src/EFCore/Properties/CoreStrings.resx

@@ -1025,7 +1025,7 @@
     <value>The specified field '{field}' could not be found for property '{2_entityType}.{1_property}'.</value>
   </data>
   <data name="MissingInverseManyToManyNavigation" xml:space="preserve">
-    <value>Unable to set up a many-to-many relationship between the entity types '{principalEntityType}' and '{declaringEntityType}' because one of the navigations was not specified. Provide a navigation in the 'HasMany' call in 'OnModelCreating'.</value>
+    <value>Unable to set up a many-to-many relationship between the entity types '{principalEntityType}' and '{declaringEntityType}' because one of the navigations was not specified. Provide a navigation in the 'HasMany' call in 'OnModelCreating'. Consider adding a private property for this.</value>
   </data>
   <data name="ModelMutable" xml:space="preserve">
     <value>Runtime metadata changes are not allowed when the model hasn't been marked as read-only.</value>
@@ -1437,6 +1437,9 @@
     <value>The entity type '{entityType}' is in shadow state. A valid model requires all entity types to have a corresponding CLR type.</value>
     <comment>Obsolete</comment>
   </data>
+  <data name="ShadowManyToManyNavigation" xml:space="preserve">
+    <value>Unable to set up a many-to-many relationship between '{leftEntityType}.{leftNavigation}' and '{rightEntityType}.{rightNavigation}' because one or both of the navigations don't have a corresponding CLR property. Consider adding a corresponding private property to the entity CLR type.</value>
+  </data>
   <data name="SharedTypeDerivedType" xml:space="preserve">
     <value>The shared-type entity type '{entityType}' cannot have a base type.</value>
   </data>

test/EFCore.Tests/ModelBuilding/ManyToManyTestBase.cs

@@ -467,6 +467,29 @@ public virtual void Throws_for_many_to_many_with_only_one_navigation_configured(
                             .WithMany(d => d.ManyToManyPrincipals)).Message);
             }
 
+            [ConditionalFact]
+            public virtual void Throws_for_many_to_many_with_a_shadow_navigation()
+            {
+                var modelBuilder = CreateModelBuilder();
+
+                modelBuilder.Ignore<OneToOneNavPrincipal>();
+                modelBuilder.Ignore<OneToManyNavPrincipal>();
+                modelBuilder.Entity<NavDependent>().Ignore(d => d.ManyToManyPrincipals);
+
+                modelBuilder.Entity<ManyToManyNavPrincipal>()
+                            .HasMany(d => d.Dependents)
+                            .WithMany("Shadow");
+
+                Assert.Equal(
+                    CoreStrings.ShadowManyToManyNavigation(
+                        nameof(NavDependent),
+                        "Shadow",
+                        nameof(ManyToManyNavPrincipal),
+                        nameof(ManyToManyNavPrincipal.Dependents)),
+                    Assert.Throws<InvalidOperationException>(
+                        () => modelBuilder.FinalizeModel()).Message);
+            }
+
             [ConditionalFact]
             public virtual void Throws_for_self_ref_with_same_navigation()
             {

test/EFCore.Tests/ModelBuilding/ModelBuilderGenericRelationshipStringTest.cs

@@ -28,6 +28,12 @@ protected override TestModelBuilder CreateTestModelBuilder(TestHelpers testHelpe
                 => new GenericStringTestModelBuilder(testHelpers, configure);
         }
 
+        public class GenericManyToManyString : ManyToManyTestBase
+        {
+            protected override TestModelBuilder CreateTestModelBuilder(TestHelpers testHelpers, Action<ModelConfigurationBuilder>? configure)
+                => new GenericStringTestModelBuilder(testHelpers, configure);
+        }
+
         public class GenericOneToOneString : OneToOneTestBase
         {
             protected override TestModelBuilder CreateTestModelBuilder(TestHelpers testHelpers, Action<ModelConfigurationBuilder>? configure)