Raise if camelcase inflection impossible in generator

When passing a camelcased model name to a generator sometimes it's
impossible to inflect the casing.  For example "BFF" will generate a
route for the controller BffsController, but the controller generated is
BfFsController.

The casing is impossible to inflect if both:
- a camelcased model name is passed to a generator
- `name.pluralize.underscore.singularize != name.underscore.singularize`

If this happens we raise an error. The error message explains the user
can either us an underscored name or add an inflection instead.

railties/lib/rails/generators/model_helpers.rb

@@ -11,6 +11,11 @@ module Rails
       [WARNING] Rails cannot recover singular form from its plural form '%s'.
       Please setup custom inflection rules for this noun before running the generator in config/initializers/inflections.rb.
       WARNING
+      INFLECTION_IMPOSSIBLE_ERROR_MESSAGE = <<~ERROR
+      Rails cannot recover the underscored form from its camelcase form '%s'.
+      Please use an underscored name instead, either '%s' or '%s'.
+      Or setup custom inflection rules for this noun before running the generator in config/initializers/inflections.rb.
+      ERROR
       mattr_accessor :skip_warn
 
       def self.included(base) #:nodoc:
@@ -19,7 +24,7 @@ module Rails
 
       def initialize(args, *_options)
         super
-        if name == name.pluralize && name.singularize != name.pluralize && !options[:force_plural]
+        if plural_model_name?(name) && !options[:force_plural]
           singular = name.singularize
           unless ModelHelpers.skip_warn
             say PLURAL_MODEL_NAME_WARN_MESSAGE % [name, singular]
@@ -27,11 +32,30 @@ module Rails
           name.replace singular
           assign_names!(name)
         end
-        if name.singularize != name.pluralize.singularize && ! ModelHelpers.skip_warn
+        if inflection_impossible?(name)
+          option1 = name.singularize.underscore
+          option2 = name.pluralize.underscore.singularize
+          raise Error, INFLECTION_IMPOSSIBLE_ERROR_MESSAGE % [name, option1, option2]
+        end
+        if irregular_model_name?(name) && ! ModelHelpers.skip_warn
           say IRREGULAR_MODEL_NAME_WARN_MESSAGE % [name.pluralize]
         end
         ModelHelpers.skip_warn = true
       end
+
+      private
+        def plural_model_name?(name)
+          name == name.pluralize && name.singularize != name.pluralize
+        end
+
+        def irregular_model_name?(name)
+          name.singularize != name.pluralize.singularize
+        end
+
+        def inflection_impossible?(name)
+          name != name.underscore &&
+            name.singularize.underscore != name.pluralize.underscore.singularize
+        end
     end
   end
 end

railties/test/generators/model_generator_test.rb

@@ -68,6 +68,16 @@ class ModelGeneratorTest < Rails::Generators::TestCase
     assert_no_match("[WARNING] Rails cannot recover singular form from its plural form", regular_content)
   end
 
+  def test_impossible_inflection_rules_raises_an_error
+    content = capture(:stderr) { run_generator ["BFF"] }
+    message = <<~MESSAGE
+      Rails cannot recover the underscored form from its camelcase form 'BFF'.
+      Please use an underscored name instead, either 'bff' or 'bf_f'.
+      Or setup custom inflection rules for this noun before running the generator in config/initializers/inflections.rb.
+    MESSAGE
+    assert_match message, content
+  end
+
   def test_model_with_underscored_parent_option
     run_generator ["account", "--parent", "admin/account"]
     assert_file "app/models/account.rb", /class Account < Admin::Account/