Consistently merge precisions in Calendar subsecond operations, closes #12303

Author: josevalim

lib/elixir/lib/calendar/datetime.ex

@@ -1493,6 +1493,13 @@ defmodule DateTime do
       iex> dt |> DateTime.add(1, :day, FakeTimeZoneDatabase)
       #DateTime<2019-04-01 02:00:00+02:00 CEST Europe/Copenhagen>
 
+  This operation merges the precision of the naive date time with the given unit:
+
+      iex> result = DateTime.add(~U[2014-10-02 00:29:10Z], 21, :millisecond)
+      ~U[2014-10-02 00:29:10.021Z]
+      iex> result.microsecond
+      {21000, 3}
+
   """
   @doc since: "1.8.0"
   @spec add(
@@ -1537,6 +1544,7 @@ defmodule DateTime do
 
     ppd = System.convert_time_unit(86400, :second, unit)
     total_offset = System.convert_time_unit(utc_offset + std_offset, :second, unit)
+    precision = max(Calendar.ISO.time_unit_to_precision(unit), precision)
 
     result =
       datetime

lib/elixir/lib/calendar/iso.ex

@@ -247,6 +247,17 @@ defmodule Calendar.ISO do
   defguardp is_utc_offset(offset) when is_integer(offset)
   defguardp is_std_offset(offset) when is_integer(offset)
 
+  @doc """
+  Converts a `t:System.time_unit/0` to precision.
+
+  Integer-based time units always get maximum precision.
+  """
+  def time_unit_to_precision(:nanosecond), do: 6
+  def time_unit_to_precision(:microsecond), do: 6
+  def time_unit_to_precision(:millisecond), do: 3
+  def time_unit_to_precision(:second), do: 0
+  def time_unit_to_precision(_), do: 6
+
   @doc """
   Parses a time `string` in the `:extended` format.
 

lib/elixir/lib/calendar/naive_datetime.ex

@@ -379,7 +379,7 @@ defmodule NaiveDateTime do
   It can also work with subsecond precisions:
 
       iex> NaiveDateTime.add(~N[2014-10-02 00:29:10], 2_000, :millisecond)
-      ~N[2014-10-02 00:29:12]
+      ~N[2014-10-02 00:29:12.000]
 
   As well as days/hours/minutes:
 
@@ -390,16 +390,12 @@ defmodule NaiveDateTime do
       iex> NaiveDateTime.add(~N[2015-02-28 00:29:10], 60, :minute)
       ~N[2015-02-28 01:29:10]
 
-  This operation keeps the precision of the naive date time:
-
-      iex> NaiveDateTime.add(~N[2014-10-02 00:29:10.021], 21, :second)
-      ~N[2014-10-02 00:29:31.021]
-
-  And ignores any changes below the precision:
+  This operation merges the precision of the naive date time with the given unit:
 
-      iex> hidden = NaiveDateTime.add(~N[2014-10-02 00:29:10], 21, :millisecond)
-      iex> hidden.microsecond # ~N[2014-10-02 00:29:10]
-      {21000, 0}
+      iex> result = NaiveDateTime.add(~N[2014-10-02 00:29:10], 21, :millisecond)
+      ~N[2014-10-02 00:29:10.021]
+      iex> result.microsecond
+      {21000, 3}
 
   Operations on top of gregorian seconds or the Unix epoch are optimized:
 
@@ -440,6 +436,7 @@ defmodule NaiveDateTime do
       )
       when is_integer(amount_to_add) do
     ppd = System.convert_time_unit(86400, :second, unit)
+    precision = max(Calendar.ISO.time_unit_to_precision(unit), precision)
 
     naive_datetime
     |> to_iso_days()

lib/elixir/lib/calendar/time.ex

@@ -493,6 +493,13 @@ defmodule Time do
       iex> Time.add(~T[17:10:05], 30, :minute)
       ~T[17:40:05]
 
+  This operation merges the precision of the time with the given unit:
+
+      iex> result = Time.add(~T[00:29:10], 21, :millisecond)
+      ~T[00:29:10.021]
+      iex> result.microsecond
+      {21000, 3}
+
   """
   @doc since: "1.6.0"
   @spec add(Calendar.time(), integer, :hour | :minute | System.time_unit()) :: t
@@ -511,6 +518,7 @@ defmodule Time do
     amount_to_add = System.convert_time_unit(amount_to_add, unit, :microsecond)
     total = time_to_microseconds(time) + amount_to_add
     parts = Integer.mod(total, @parts_per_day)
+    precision = max(Calendar.ISO.time_unit_to_precision(unit), precision)
 
     {hour, minute, second, {microsecond, _}} =
       calendar.time_from_day_fraction({parts, @parts_per_day})
@@ -736,14 +744,14 @@ defmodule Time do
           hour: hour1,
           minute: minute1,
           second: second1,
-          microsecond: {microsecond1, @parts_per_day}
+          microsecond: {microsecond1, _}
         },
         %{
           calendar: Calendar.ISO,
           hour: hour2,
           minute: minute2,
           second: second2,
-          microsecond: {microsecond2, @parts_per_day}
+          microsecond: {microsecond2, _}
         },
         unit
       ) do