entity repro1 is
end;

architecture behav of repro1 is
  constant word_len : natural := 20;
  
  subtype word is bit_vector(word_len - 1 downto 0);
  function F return word is 
  begin
    return ("1010","101010101010", others => '0');
  end function;
begin
  assert f = b"1010_101010101010_0000";
end;