Cannot rollback migrations that use add_index with PostgreSQL expressions instead of column name

[oliverguenther]

Steps to reproduce

rails new cannot_rollback --database=postgresql
cd cannot_rollback
rails g migration CreateJsonTable

Open the generated migration, paste this migration into it

class CreateJsonTable < ActiveRecord::Migration[6.1]
  def change
    create_table :json_tables do |t|
      t.jsonb :data
      t.timestamps
    end

    add_index :json_tables,
              "(data->'foo')",
              using: :gin,
              name: 'index_json_tables_data_foo'
  end
end

Then run rails db:create db:migrate which runs through

Trying to roll back that index:

rails db:rollback

results in

== 20210929121223 CreateJsonTable: reverting ==================================
-- remove_index(:json_tables, "(data->'foo')", {:using=>:gin, :name=>"index_json_tables_data_foo"})
rails aborted!
StandardError: An error has occurred, this and all later migrations canceled:

No indexes found on json_tables with the options provided.
bin/rails:5:in `<top (required)>'
bin/spring:10:in `block in <top (required)>'
bin/spring:7:in `tap'
bin/spring:7:in `<top (required)>'

Expected behavior

The migration can be rolled back, especially since an explicit index name has been provded.

Actual behavior

As the expression is provided as column_name , https://github.com/rails/rails/blob/main/activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb#L1433 does not allow the index to be removed by name.

The check in this line then fails to match the index https://github.com/rails/rails/blob/main/activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb#L1447

What reasons would there be for an index not to be removable by its name if one is explictly given in the add_index definition? I'll gladly submit a PR that will skip this check if a name has been given: https://github.com/rails/rails/blob/main/activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb#L1446-L1448

System configuration

Rails version:
tested with current version 6.1

Ruby version:
irrelevant, but tested with 2.7.x