Add offense to Lint::FormatParameterMismatch about invalid format string

docs/modules/ROOT/pages/cops_lint.adoc

@@ -1100,6 +1100,10 @@ This lint sees if there is a mismatch between the number of
 expected fields for format/sprintf/#% and what is actually
 passed as arguments.
 
+In addition it checks whether different formats are used in the same
+format string. Do not mix numbered, unnumbered, and named formats in
+the same format string.
+
 === Examples
 
 [source,ruby]
@@ -1116,6 +1120,20 @@ format('A value: %s and another: %i', a_value)
 format('A value: %s and another: %i', a_value, another)
 ----
 
+[source,ruby]
+----
+# bad
+
+format('Unnumbered format: %s and numbered: %2$s', a_value, another)
+----
+
+[source,ruby]
+----
+# good
+
+format('Numbered format: %1$s and numbered %2$s', a_value, another)
+----
+
 == Lint/HeredocMethodCallPosition
 
 |===

lib/rubocop/cop/lint/format_parameter_mismatch.rb

@@ -7,6 +7,10 @@ module Lint
       # expected fields for format/sprintf/#% and what is actually
       # passed as arguments.
       #
+      # In addition it checks whether different formats are used in the same
+      # format string. Do not mix numbered, unnumbered, and named formats in
+      # the same format string.
+      #
       # @example
       #
       #   # bad
@@ -18,32 +22,58 @@ module Lint
       #   # good
       #
       #   format('A value: %s and another: %i', a_value, another)
+      #
+      # @example
+      #
+      #   # bad
+      #
+      #   format('Unnumbered format: %s and numbered: %2$s', a_value, another)
+      #
+      # @example
+      #
+      #   # good
+      #
+      #   format('Numbered format: %1$s and numbered %2$s', a_value, another)
       class FormatParameterMismatch < Cop
         # http://rubular.com/r/CvpbxkcTzy
         MSG = "Number of arguments (%<arg_num>i) to `%<method>s` doesn't " \
               'match the number of fields (%<field_num>i).'
+        MSG_INVALID = 'Format string is invalid because formatting sequence types ' \
+                      '(numbered, named or unnumbered) are mixed.'
 
         KERNEL = 'Kernel'
         SHOVEL = '<<'
         STRING_TYPES = %i[str dstr].freeze
 
         def on_send(node)
+          return unless format_string?(node)
+
+          if invalid_format_string?(node)
+            add_offense(node, location: :selector, message: MSG_INVALID)
+            return
+          end
+
           return unless offending_node?(node)
 
           add_offense(node, location: :selector)
         end
 
         private
 
+        def format_string?(node)
+          called_on_string?(node) && method_with_format_args?(node)
+        end
+
+        def invalid_format_string?(node)
+          !RuboCop::Cop::Utils::FormatString.new(node.source).valid?
+        end
+
         def offending_node?(node)
-          return false unless called_on_string?(node)
-          return false unless method_with_format_args?(node)
           return false if splat_args?(node)
 
           num_of_format_args, num_of_expected_fields = count_matches(node)
 
           return false if num_of_format_args == :unknown
-          return false if num_of_expected_fields == :unknown
 
           matched_arguments_count?(num_of_expected_fields, num_of_format_args)
         end
@@ -114,8 +144,6 @@ def expected_fields_count(node)
           return :unknown unless node.str_type?
 
           format_string = RuboCop::Cop::Utils::FormatString.new(node.source)
-
-          return :unknown unless format_string.valid?
           return 1 if format_string.named_interpolation?
 
           max_digit_dollar_num = format_string.max_digit_dollar_num

spec/rubocop/cop/lint/format_parameter_mismatch_spec.rb

@@ -172,8 +172,11 @@
   end
 
   context 'when format is invalid' do
-    it 'does not register an offense' do
-      expect_no_offenses("format('%s %2$s', 'foo', 'bar')")
+    it 'registers an offense' do
+      expect_offense(<<~RUBY)
+        format('%s %2$s', 'foo', 'bar')
+        ^^^^^^ Format string is invalid because formatting sequence types (numbered, named or unnumbered) are mixed.
+      RUBY
     end
   end