[release/8.0] Avoid infinite recursion on identifying shadow FKs (#34891)

Fixes #34875

Author: AndriySvyryd

src/EFCore/Metadata/Conventions/ForeignKeyPropertyDiscoveryConvention.cs

@@ -50,6 +50,9 @@ public class ForeignKeyPropertyDiscoveryConvention :
     IPropertyFieldChangedConvention,
     IModelFinalizingConvention
 {
+    private static readonly bool UseOldBehavior34875 =
+        AppContext.TryGetSwitch("Microsoft.EntityFrameworkCore.Issue34875", out var enabled34875) && enabled34875;
+
     /// <summary>
     ///     Creates a new instance of <see cref="ForeignKeyPropertyDiscoveryConvention" />.
     /// </summary>
@@ -142,7 +145,7 @@ private IConventionForeignKeyBuilder ProcessForeignKey(
                     && fkProperty.ClrType.IsNullableType() == foreignKey.IsRequired
                     && fkProperty.GetContainingForeignKeys().All(otherFk => otherFk.IsRequired == foreignKey.IsRequired))
                 {
-                    var newType = fkProperty.ClrType.MakeNullable(!foreignKey.IsRequired);
+                    var newType = fkProperty.ClrType.MakeNullable(!foreignKey.IsRequired && (!fkProperty.IsKey() || UseOldBehavior34875));
                     if (fkProperty.ClrType != newType)
                     {
                         fkProperty.DeclaringType.Builder.Property(

test/EFCore.Specification.Tests/ModelBuilding101ManyToManyTestBase.cs

@@ -737,6 +737,61 @@ protected override void OnModelCreating(ModelBuilder modelBuilder)
         }
     }
 
+    [ConditionalFact]
+    public virtual void ManyToManyWithPayloadAndNavsToJoinClassShadowFKsTest()
+        => Model101Test();
+
+    protected class ManyToManyWithPayloadAndNavsToJoinClassShadowFKs
+    {
+        public class Post
+        {
+            public int Id { get; set; }
+            public List<Tag> Tag { get; } = [];
+            public List<PostTag> PostTags { get; } = [];
+        }
+
+        public class Tag
+        {
+            public int Id { get; set; }
+            public List<Post> Post { get; } = [];
+            public List<PostTag> PostTags { get; } = [];
+        }
+
+        public class PostTag
+        {
+        }
+
+        public class Context0 : Context101
+        {
+            public DbSet<Post> Post
+                => Set<Post>();
+
+            public DbSet<Tag> Tag
+                => Set<Tag>();
+
+            protected override void OnModelCreating(ModelBuilder modelBuilder)
+                => modelBuilder.Entity<Post>()
+                    .HasMany(e => e.Tag)
+                    .WithMany(e => e.Post)
+                    .UsingEntity<PostTag>(
+                        l => l.HasOne<Tag>().WithMany(t => t.PostTags),
+                        r => r.HasOne<Post>().WithMany(p => p.PostTags),
+                        j => { });
+        }
+
+        public class Context1 : Context0
+        {
+            protected override void OnModelCreating(ModelBuilder modelBuilder)
+                => modelBuilder.Entity<Post>()
+                    .HasMany(e => e.Tag)
+                    .WithMany(e => e.Post)
+                    .UsingEntity<PostTag>(
+                        l => l.HasOne<Tag>().WithMany(t => t.PostTags).HasForeignKey("TagId"),
+                        r => r.HasOne<Post>().WithMany(p => p.PostTags).HasForeignKey("PostId"),
+                        j => { });
+        }
+    }
+
     [ConditionalFact]
     public virtual void ManyToManyWithNoCascadeDeleteTest()
         => Model101Test();