Historically Hibernate always expects that @GeneratedValue is matched with a matching named "generator definition". The name is provided by GeneratedValue#generator. The corresponding "generator definition" would be a @GenericGenerator, @SequenceGenerator or @TableGenerator based on the value of GeneratedValue#strategy:

SEQUENCE and TABLE

Given @GeneratedValue( strategy=SEQUENCE, ... ) Hibernate would look for a matching @GenericGenerator or a @SequenceGenerator.
Given @GeneratedValue( strategy=TABLE, ... ) Hibernate would look for a matching @GenericGenerator or a @TableGenerator.

Again, it would be an error if Hibernate did not find the matching @GenericGenerator, @SequenceGenerator or @TableGenerator. Ultimately though, these generator definitions are really just about handling non-default situations (e.g., use a specific increment size for a sequence/table, etc). But it should be ok for the user to leave them off and just get the "default behavior" (e.g. increment size of 50 as JPA defines for @SequenceGenerator and @TableGenerator).

In other words, this should be a valid mapping:

Here we would pick up the JPA defined defaults (see @SequenceGenerator and @TableGenerator for the defined defaults).

The name is an additional discussion, and is the same discussion for:

In both cases we do not provide an explicit @SequenceGenerator#sequenceName value. Historically Hibernate interprets the default name for both sequence and table as "hibernate_sequence". Ideally though (keeping with the theme of @SequenceGenerator and @TableGenerator defining just overrides) we'd want the sequence here to be named "my_db_sequence".

So another part of the work here is to support this distinction - by a setting hibernate.model.generator_name_as_sequence_name. By default, this is
true which means we will pick "my_db_sequence" (@GeneratedValue#generator)); setting that to false indicates to pick the legacy "hibernate_sequence" name.

IDENTITY

This discussion has no bearing on how GenerationType#IDENTITY is handled.

AUTO

In keeping with "less typing == better", this also covers some cases related to GenerationType#AUTO specifically in how org.hibernate.boot.model.IdGeneratorStrategyInterpreter#determineGeneratorName can operate. This work changes the "parameter object" passed in to that method to give it access to the @GeneratedValue#generator value. At the moment, this only case this is used is to handle:

Previously this mapping would not work. Instead a user would have to say:

"increment" generator is a Hibernate-specific, in-memory strategy performing simple incrementing for generating id values. Specifically it is a short-name for Hibernate's org.hibernate.id.IncrementGenerator. Generally speaking there is little need to configure IncrementGenerator, so requiring the @GenericGenerator is unnecessary in most cases.