Improve did_you_mean partial name correction

Before this change did_you_mean shows partial paths like `animals/_partial`,
but adding that to your render call in a view like `<%= render 'animals/_partial' %>`
 will still be missing as rails will search for the template `animals/__partial`.
We can provide the user a easier copy/paste correction if we don't tell them about the underscore.

This also reduces the candidates to only return partials if you are
looking for a partial and only look for non-templates when you are not
looking for a template.

actionview/lib/action_view/template/error.rb

@@ -27,7 +27,7 @@ module ActionView
   end
 
   class MissingTemplate < ActionViewError #:nodoc:
-    attr_reader :path, :paths, :prefixes
+    attr_reader :path, :paths, :prefixes, :partial
 
     def initialize(paths, path, prefixes, partial, details, *)
       if partial && path.present?
@@ -37,6 +37,7 @@ module ActionView
       @path = path
       @paths = paths
       @prefixes = Array(prefixes)
+      @partial = partial
       template_type = if partial
         "partial"
       elsif /layouts/i.match?(path)
@@ -93,7 +94,13 @@ module ActionView
       def corrections
         path = @error.path
         prefixes = @error.prefixes
+
         candidates = @error.paths.flat_map(&:all_template_paths).uniq
+        if @error.partial
+          candidates = candidates.grep(%r{_[^/]+\z})
+        else
+          candidates = candidates.grep_v(%r{_[^/]+\z})
+        end
 
         # Group by possible prefixes
         files_by_dir = candidates.group_by do |x|
@@ -135,7 +142,11 @@ module ActionView
           end
         end
 
-        results.to_a
+        if @error.partial
+          results.to_a.map { |res| res.sub(%r{_([^/]+)\z}, "\\1") }
+        else
+          results.to_a
+        end
       end
     end
 

actionview/test/template/render_test.rb

@@ -626,13 +626,27 @@ module RenderTestCases
   if defined?(DidYouMean) && DidYouMean.respond_to?(:correct_error)
     def test_render_partial_provides_spellcheck
       e = assert_raises(ActionView::MissingTemplate) { @view.render(partial: "test/partail") }
-      assert_match %r{Did you mean\?  test/_partial\n *test/_partialhtml}, e.message
+      assert_match %r{Did you mean\?  test/partial\n *test/partialhtml}, e.message
     end
 
     def test_spellcheck_doesnt_list_directories
       e = assert_raises(ActionView::MissingTemplate) { @view.render(partial: "test/directory") }
       assert_match %r{Did you mean\?}, e.message
-      assert_no_match %r{Did you mean\?  test/_directory\n}, e.message # test/hello is a directory
+      assert_no_match %r{Did you mean\?  test/directory\n}, e.message # test/hello is a directory
+    end
+
+    def test_spellcheck_only_lists_templates
+      e = assert_raises(ActionView::MissingTemplate) { @view.render(template: "test/partial") }
+
+      assert_match %r{Did you mean\?}, e.message
+      assert_no_match %r{Did you mean\?  test/partial\n}, e.message
+    end
+
+    def test_spellcheck_only_lists_partials
+      e = assert_raises(ActionView::MissingTemplate) { @view.render(partial: "test/template") }
+
+      assert_match %r{Did you mean\?}, e.message
+      assert_no_match %r{Did you mean\?  test/template\n}, e.message
     end
   end