Add test to confirm crash when available locales are enforced but fal…

…lback locale unavailable

spec/grape/exceptions/base_spec.rb

@@ -0,0 +1,31 @@
+require 'spec_helper'
+
+describe Grape::Exceptions::Base do
+  describe '#compose_message' do
+    subject { described_class.new.send(:compose_message, key, attributes) }
+
+    let(:key) { :invalid_formatter }
+    let(:attributes) { { klass: String, to_format: 'xml' } }
+
+    context 'when I18n enforces available locales' do
+      before { I18n.enforce_available_locales = true }
+      after { I18n.enforce_available_locales = false }
+
+      context 'when the fallback locale is available' do
+        before { I18n.available_locales = %i[de en] }
+
+        it 'returns the translated message' do
+          expect(subject).to eq('cannot convert String to xml')
+        end
+      end
+
+      context 'when the fallback locale is not available' do
+        before { I18n.available_locales = %i[de jp] }
+
+        it 'currently raises an error' do
+          expect { subject }.to raise_error(I18n::InvalidLocale)
+        end
+      end
+    end
+  end
+end