Fix Style/BlockDelimiters for blocks with numbered arguments

With `EnforcedStyle` of `line_count_based`, the default, the following
code **will** issue an offense, and will be auto-corrected:

```ruby
[1, 2, 3].map { |n| # [1, 2, 3].map do |n|
  n + 1             #   n + 1
}                   # end

```

While the code below won't be converted to a multiline `do ... end`
block:

```ruby
[1, 2, 3].map {
  _1 + 1
}
```

This is yet another case of missing `on_numblock` implementation. I see
a dozen of those in the codebase and I think whenever we handle `block`
AST nodes we should also handle `numblock` nodes. I can go over the
codebase and fix those problems on a per-cop basis, or I can try to fix
it generally.

Currently, my best idea is to write an `InternalAffairs` cop that warns
when we define `on_block` handlers, but we miss a `on_block`
implementation. For the majority of the cases, an `on_numblock` alias of
the `on_block` method would be enough to handle blocks with numbered
arguments. I can then use the internal co and systematically go over the
cases and add aliases or custom `numblock` handlers where required. I
think this is better than solutions involving meta-programming or
default `on_numblock` implementations delegating to `on_block`.

changelog/fix_block_delimiters_with_numbered_arguments.md

@@ -0,0 +1 @@
+* [#10749](https://github.com/rubocop/rubocop/pull/10749): Fix Style/BlockDelimiters for blocks with numbered arguments. ([@gsamokovarov][])

lib/rubocop/cop/style/block_delimiters.rb

@@ -184,6 +184,8 @@ def on_block(node)
           end
         end
 
+        alias on_numblock on_block
+
         private
 
         def autocorrect(corrector, node)
@@ -300,7 +302,7 @@ def with_block?(node)
 
         def get_blocks(node, &block)
           case node.type
-          when :block
+          when :block, :numblock
             yield node
           when :send
             get_blocks(node.receiver, &block) if node.receiver

spec/rubocop/cop/style/block_delimiters_spec.rb

@@ -27,6 +27,35 @@
         }, 1
       RUBY
     end
+
+    context 'Ruby >= 2.7', :ruby27 do
+      it 'registers an offense for a single line numblock with do-end' do
+        expect_offense(<<~RUBY)
+          each do _1 end
+               ^^ Prefer `{...}` over `do...end` for single-line blocks.
+        RUBY
+      end
+
+      it 'accepts a single line numblock with braces' do
+        expect_no_offenses('each { _1 }')
+      end
+
+      it 'accepts a multi-line numblock with do-end' do
+        expect_no_offenses(<<~RUBY)
+          each do
+            _1
+          end
+        RUBY
+      end
+
+      it 'accepts a multi-line numblock that needs braces to be valid ruby' do
+        expect_no_offenses(<<~RUBY)
+          puts [1, 2, 3].map {
+            _1 * _1
+          }, 1
+        RUBY
+      end
+    end
   end
 
   context 'EnforcedStyle: semantic' do