builder = fn b ->
  ~e"""
  <div class="flex">
    <%= b.(:day, [class: "xyz"]) %>
    <%= b.(:month, [class: "abc"]) %>
    <%= b.(:year, [class: "def"]) %>
  </div>
  """
end

Note to self - trying stuff out …

some_phoenix_project$ iex -S mix
Erlang/OTP 21 [erts-10.1.3] [source] [64-bit] [smp:8:8] [ds:8:8:10] [async-threads:1] [hipe] [dtrace]

Interactive Elixir (1.7.4) - press Ctrl+C to exit (type h() ENTER for help)
iex(1)> import Phoenix.HTML
Phoenix.HTML
iex(2)> import Phoenix.HTML.Form
Phoenix.HTML.Form
iex(3)> builder = fn select_for ->
...(3)>   ~E"""
...(3)>   <div class="flex">
...(3)>     <%= select_for.(:day, [class: "xyz"]) %>
...(3)>     <%= select_for.(:month, [class: "abc"]) %>
...(3)>     <%= select_for.(:year, [class: "def"]) %>
...(3)>   </div>
...(3)>   """
...(3)> end
#Function<6.128620087/1 in :erl_eval.expr/5>
iex(4)> content = ~E"""
...(4)> <%= date_select :user, :born_at,
...(4)>   builder: builder,
...(4)>   year: [options: 2018..2018],
...(4)>   month: [options: 12..12],
...(4)>   day: [options: 13..13]
...(4)> %>
...(4)> """
{:safe,
 [
   [
     "",
     [
       [
         [
           [
             [
               ["" | "<div class=\"flex\">\n  "],
               60,
               "select",
               [
                 [32, "class", 61, 34, "xyz", 34],
                 [32, "id", 61, 34, "user_born_at_day", 34],
                 [32, "name", 61, 34, "user[born_at][day]", 34]
               ],
               62,
               [
                 "",
                 60,
                 "option",
                 [[32, "value", 61, 34, "13", 34]],
                 62,
                 "13",
                 60,
                 47,
                 "option",
                 62
               ],
               60,
               47,
               "select",
               62
             ] |
             "\n  "
           ],
           60,
           "select",
           [
             [32, "class", 61, 34, "abc", 34],
             [32, "id", 61, 34, "user_born_at_month", 34],
             [32, "name", 61, 34, "user[born_at][month]", 34]
           ],
           62,
           [
             "",
             60,
             "option",
             [[32, "value", 61, 34, "12", 34]],
             62,
             "12",
             60,
             47,
             "option",
             62
           ],
           60,
           47,
           "select",
           62
         ] |
         "\n  "
       ],
       60,
       "select",
       [
         [32, "class", 61, 34, "def", 34],
         [32, "id", 61, 34, "user_born_at_year", 34],
         [32, "name", 61, 34, "user[born_at][year]", 34]
       ],
       62,
       [
         "",
         60,
         "option",
         [[32, "value", 61, 34, "2018", 34]],
         62,
         "2018",
         60,
         47,
         "option",
         62
       ],
       60,
       47,
       "select",
       62
     ] |
     "\n</div>\n"
   ] |
   "\n"
 ]}
iex(5)> safe_to_string(content)
"<div class=\"flex\">\n  <select class=\"xyz\" id=\"user_born_at_day\" name=\"user[born_at][day]\"><option value=\"13\">13</option></select>\n  <select class=\"abc\" id=\"user_born_at_month\" name=\"user[born_at][month]\"><option value=\"12\">12</option></select>\n  <select class=\"def\" id=\"user_born_at_year\" name=\"user[born_at][year]\"><option value=\"2018\">2018</option></select>\n</div>\n\n"
iex(6)> 

You provided this as your reference case:

My purpose was to attempt to generate equivalent HTML output with a builder function.

So if we start out with (in iex):

import Phoenix.HTML
import Phoenix.HTML.Form

default_date = {2020, 12, 25}
content =
  ~E"""
    <%= date_select :user, :date, default: default_date %>
  """

safe_to_string(content)

We’ll get this output:

<select id=\"user_date_year\" name=\"user[date][year]\">
  <option value=\"2014\">2014</option>
  <option value=\"2015\">2015</option>
  <option value=\"2016\">2016</option>
  <option value=\"2017\">2017</option>
  <option value=\"2018\">2018</option>
  <option value=\"2019\">2019</option>
  <option value=\"2020\" selected>2020</option>
  <option value=\"2021\">2021</option>
  <option value=\"2022\">2022</option>
  <option value=\"2023\">2023</option>
  <option value=\"2024\">2024</option>
</select> /
<select id=\"user_date_month\" name=\"user[date][month]\">
  <option value=\"1\">January</option>
  <option value=\"2\">February</option>
  <option value=\"3\">March</option>
  <option value=\"4\">April</option>
  <option value=\"5\">May</option>
  <option value=\"6\">June</option>
  <option value=\"7\">July</option>
  <option value=\"8\">August</option>
  <option value=\"9\">September</option>
  <option value=\"10\">October</option>
  <option value=\"11\">November</option>
  <option value=\"12\" selected>December</option>
</select> /
<select id=\"user_date_day\" name=\"user[date][day]\">
  <option value=\"1\">01</option>
  <option value=\"2\">02</option>
  <option value=\"3\">03</option>
  <option value=\"4\">04</option>
  <option value=\"5\">05</option>
  <option value=\"6\">06</option>
  <option value=\"7\">07</option>
  <option value=\"8\">08</option>
  <option value=\"9\">09</option>
  <option value=\"10\">10</option>
  <option value=\"11\">11</option>
  <option value=\"12\">12</option>
  <option value=\"13\">13</option>
  <option value=\"14\">14</option>
  <option value=\"15\">15</option>
  <option value=\"16\">16</option>
  <option value=\"17\">17</option>
  <option value=\"18\">18</option>
  <option value=\"19\">19</option>
  <option value=\"20\">20</option>
  <option value=\"21\">21</option>
  <option value=\"22\">22</option>
  <option value=\"23\">23</option>
  <option value=\"24\">24</option>
  <option value=\"25\" selected>25</option>
  <option value=\"26\">26</option>
  <option value=\"27\">27</option>
  <option value=\"28\">28</option>
  <option value=\"29\">29</option>
  <option value=\"30\">30</option>
  <option value=\"31\">31</option>
</select>\n"

Note how the “2020”, “12” and “25” option elements have the selected attribute.

Now a builder version:

import Phoenix.HTML
import Phoenix.HTML.Form

make_builder = fn {year, month, day} ->
  fn b ->
    ~E"""
      <%= b.(:year, [value: year, options: 2014..2024, class: "def"]) %> / <%= b.(:month, [value: month]) %> / <%= b.(:day, [value: day]) %>
    """
  end
end

default_date = {2020, 12, 25}
builder = make_builder.(default_date)
content =
  ~E"""
    <%= date_select :user, :date, builder: builder %>
  """

safe_to_string(content)

which produces the same output.

EDIT:

Now having seen this code:

https://github.com/phoenixframework/phoenix_html/blob/v2.13.0/lib/phoenix_html/form.ex#L1391-L1395

This works as expected:

import Phoenix.HTML
import Phoenix.HTML.Form

builder =
  fn b ->
    ~E"""
      <%= b.(:year, [options: 2014..2024, class: "def"]) %> / <%= b.(:month, []) %> / <%= b.(:day, []) %>
    """
  end

default_date = {2020, 12, 25}
content =
  ~E"""
    <%= date_select :user, :date, builder: builder, default: default_date %>
  """

safe_to_string(content)

If you look at the date_select code it should become obvious that the :default value has already been extracted and is being handed to the builder function as the value to format.

By the time builder runs it’s too late to influence the :default because

value = Keyword.get(opts, :value, input_value(form, field) || Keyword.get(opts, :default))

has already happened.