Deprecate fallback connection to determine platform

UPGRADE.md

@@ -8,6 +8,12 @@ awareness about deprecated code.
 
 # Upgrade to 3.5
 
+## Deprecated fallback connection used to determine the database platform.
+
+Relying on a fallback connection used to determine the database platform while connecting to a non-existing database
+has been deprecated. Either use an existing database name in connection parameters or omit the database name
+if the platform and the server configuration allow that.
+
 ## Deprecated default PostgreSQL connection database.
 
 Relying on the DBAL connecting to the "postgres" database by default is deprecated. Unless you want to have the server

src/Connection.php

@@ -400,6 +400,15 @@ private function getDatabasePlatformVersion()
                     throw $originalException;
                 }
 
+                Deprecation::trigger(
+                    'doctrine/dbal',
+                    'https://github.com/doctrine/dbal/pull/5707',
+                    'Relying on a fallback connection used to determine the database platform while connecting'
+                        . ' to a non-existing database is deprecated. Either use an existing database name in'
+                        . ' connection parameters or omit the database name if the platform'
+                        . ' and the server configuration allow that.',
+                );
+
                 // The database to connect to might not yet exist.
                 // Retry detection without database name connection parameter.
                 $params = $this->params;