remove unused variable and assignment

remove unused match variables in selector

fix wrong argument error message [ci skip]

Introduced in 59d20af2.

Add a note about getting the "no layout" behavior by returning "false"
to make it easier for people that might need to change their code.

Related to #8458. [ci skip]

This way the available options are only built for actually fetching the
algorithm in case the option is given.

The options are going to be necessary a second time only in case the option
is given but does not exist, which is supposed to be due to a typo or
something like that, so no problem.

Also remove duplicated tests for Errors#as_json and minor improvements
in some tests.

Add a working example of how to use this new method.

When building the indexes list in PostgreSQL, IndexDefinition objects
are only created if the query for the related attributes really returns
something matching the attributes.

In case it does not, the variables for building the definition objects were
being created but not used.

improve readability

Refactored to remove unnecessary setup

Reorganize scoping test files

Change merge to merge! to avoid creating new objects

Fix some typos in AS

Do the absolute bare minimum needed to define this task, no need to compute
gem paths everytime you call rake.

This code is executed every time you run a rake task, there is no need to, specially
adding a penalty to test suites.

I see no need to list the Gemfile contents (twice!) to convey this
message.

Conflicts:
	activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb
	activerecord/test/cases/adapter_test.rb
	guides/source/testing.md

[ci skip]

The file name should be name_scoping_test.rb and the class should be
`NamedScopingTest` according to ActiveRecord::Scoping::Name

The scoping/default.rb and scoping/named.rb got moved under scoping/ in
commit 2b22564c,
but the tests never did.