[Fix rubocop#12843] Fix an error for Lint/MixedCaseRange

Fixes rubocop#12843.

This PR fixes an error for `Lint/MixedCaseRange`
when a character between `Z` and `a` is used in the regexp range.

changelog/fix_an_error_for_lint_mixed_case_range.md

@@ -0,0 +1 @@
+* [#12843](https://github.com/rubocop/rubocop/issues/12843): Fix an error for `Lint/MixedCaseRange` when a character between `Z` and `a` is used in the regexp range. ([@koic][])

lib/rubocop/cop/lint/mixed_case_range.rb

@@ -47,8 +47,10 @@ def on_irange(node)
 
         def on_regexp(node)
           each_unsafe_regexp_range(node) do |loc|
+            next unless (replacement = regexp_range(loc.source))
+
             add_offense(loc) do |corrector|
-              corrector.replace(loc, rewrite_regexp_range(loc.source))
+              corrector.replace(loc, replacement)
             end
           end
         end
@@ -99,10 +101,13 @@ def skip_range?(range_start, range_end)
           end
         end
 
-        def rewrite_regexp_range(source)
+        def regexp_range(source)
           open, close = source.split('-')
-          first = [open, range_for(open).end]
-          second = [range_for(close).begin, close]
+          return unless (open_range = range_for(open))
+          return unless (close_range = range_for(close))
+
+          first = [open, open_range.end]
+          second = [close_range.begin, close]
           "#{first.uniq.join('-')}#{second.uniq.join('-')}"
         end
       end

spec/rubocop/cop/lint/mixed_case_range_spec.rb

@@ -228,6 +228,18 @@
     RUBY
   end
 
+  it 'does not register an offense when a character between `Z` and `a` is at the start of range.' do
+    expect_no_offenses(<<~RUBY)
+      foo = /[_-a]/
+    RUBY
+  end
+
+  it 'does not register an offense when a character between `Z` and `a` is at the end of range.' do
+    expect_no_offenses(<<~RUBY)
+      foo = /[A-_]/
+    RUBY
+  end
+
   it 'does not register an offense with invalid byte sequence in UTF-8' do
     expect_no_offenses(<<~'RUBY')
       foo = /[\–]/