Raise an ArgumentError when `include_blank` is false for a required field in

`Tags::Base#select_content_tag`.

Previously, passing a falsey value to `include_blank` would be ignored if the
field was required, and a blank line would still be inserted. The following will
now raise instead of quietly failing:

`select("post", "category", %w(a required field), { include_blank: false }, required: 'required')`

1 files changed, 9 insertions, 3 deletions

diff --git a/actionview/lib/action_view/helpers/tags/base.rb b/actionview/lib/action_view/helpers/tags/base.rb
index acc6443a96..d57f26ba4f 100644
--- a/actionview/lib/action_view/helpers/tags/base.rb
+++ b/actionview/lib/action_view/helpers/tags/base.rb
@@ -120,7 +120,12 @@ module ActionView
         def select_content_tag(option_tags, options, html_options)
           html_options = html_options.stringify_keys
           add_default_name_and_id(html_options)
-          options[:include_blank] ||= true unless options[:prompt] || select_not_required?(html_options)
+
+          if placeholder_required?(html_options)
+            raise ArgumentError, "include_blank cannot be false for a required field." if options[:include_blank] == false
+            options[:include_blank] ||= true unless options[:prompt]
+          end
+
           value = options.fetch(:selected) { value(object) }
           select = content_tag("select", add_options(option_tags, options, value), html_options)
 
@@ -131,8 +136,9 @@ module ActionView
           end
         end
 
-        def select_not_required?(html_options)
-          !html_options["required"] || html_options["multiple"] || html_options["size"].to_i > 1
+        def placeholder_required?(html_options)
+          # See https://html.spec.whatwg.org/multipage/forms.html#attr-select-required
+          html_options["required"] && !html_options["multiple"] && html_options.fetch("size", 1).to_i == 1
         end
 
         def add_options(option_tags, options, value = nil)