MySQL does not support `onDelete: setDefault`

When trying to migrate a super simple schema to MySQL with an onDelete: setDefault, this happens:

C:\Users\Jan\Documents\throwaway\setdefault>npx prisma db push     
Environment variables loaded from .env
Prisma schema loaded from prisma\schema.prisma
Datasource "db": MySQL database "purple_kingfisher" at "mysql-db-provision.cm0mkpwj8arx.eu-central-1.rds.amazonaws.com:3306"
Error: Cannot add foreign key constraint
   0: sql_migration_connector::sql_database_step_applier::apply_migration
             at migration-engine\connectors\sql-migration-connector\src\sql_database_step_applier.rs:11
   1: migration_core::api::SchemaPush
             at migration-engine\core\src\api.rs:187

model OnDeleteSetDefaultParent {
  id                Int                                @id @default(autoincrement())
  name              String                             @unique
  mandatoryChildren OnDeleteSetDefaultMandatoryChild[]
}

model OnDeleteSetDefaultMandatoryChild {
  id       Int                      @id @default(autoincrement())
  name     String                   @unique
  parent   OnDeleteSetDefaultParent @relation(fields: [parentId], references: [id], onDelete: SetDefault)
  parentId Int                      @default(1)
}

Per our documentation this should fundamentally work (if maybe a bit different than expected):

But looking at the MySQL docs, this does not seem to be supported at all:

SET DEFAULT: This action is recognized by the MySQL parser, but both InnoDB and NDB reject table definitions containing ON DELETE SET DEFAULT or ON UPDATE SET DEFAULT clauses.

• https://dev.mysql.com/doc/refman/8.0/en/create-table-foreign-keys.html#:~:text=SET DEFAULT%3A This action is recognized by the MySQL parser%2C but both InnoDB and NDB reject table definitions containing ON DELETE SET DEFAULT or ON UPDATE SET DEFAULT clauses.
• https://dev.mysql.com/doc/refman/5.7/en/create-table-foreign-keys.html#:~:text=SET DEFAULT%3A This action is recognized by the MySQL parser%2C but both InnoDB and NDB reject table definitions containing ON DELETE SET DEFAULT or ON UPDATE SET DEFAULT clauses.

Similar for MariaDB:

The SET DEFAULT action is not supported.

• https://mariadb.com/kb/en/foreign-keys/#:~:text=The SET DEFAULT action is not supported.

We even mention similar in our engines comments: https://github.com/prisma/prisma-engines/blob/ccf3dc944acdabb431947150e12b984b34c538cd/query-engine/connector-test-kit-rs/query-engine-tests/tests/new/ref_actions/on_delete/set_default.rs#L1 https://github.com/prisma/prisma-engines/blob/ccf3dc944acdabb431947150e12b984b34c538cd/migration-engine/migration-engine-tests/tests/migrations/relations.rs#L565-L566

We should adapt our validation to not allow this, and update our documentation afterwards as well. No need for our users to waste their time with this.

[jkomyno]

I have summarised my findings in this internal notion doc.

[jkomyno]

I have just found out that MySQL 8.0 docs report the following:

SET DEFAULT: This action is recognized by the MySQL parser, but both InnoDB and NDB reject table definitions containing ON DELETE SET DEFAULT or ON UPDATE SET DEFAULT clauses.