Define function composition for date/time values (#814)

* Define function composition for date/time values

* Apply suggestions from code review

Co-authored-by: Stanisław Małolepszy <sta@malolepszy.org>

* Drop the "only"

* Update spec/registry.md

* Update spec/registry.md

Co-authored-by: Eemeli Aro <eemeli@mozilla.com>

* Update spec/registry.md

Co-authored-by: Eemeli Aro <eemeli@mozilla.com>

* Update spec/registry.md

Co-authored-by: Eemeli Aro <eemeli@mozilla.com>

* Make :date and :time composition implementation-defined

---------

Co-authored-by: Stanisław Małolepszy <sta@malolepszy.org>
Co-authored-by: Addison Phillips <addison@unicode.org>

Author: 3 people

spec/registry.md

@@ -593,6 +593,11 @@ output.
 If both are specified, a _Bad Option_ error MUST be emitted
 and a _fallback value_ used as the _resolved value_ of the _expression_.
 
+If the _operand_ of the _expression_ is an implementation-defined date/time type,
+it can include _style options_, _field options_, or other option values.
+These are included in the resolved option values of the _expression_,
+with _options_ on the _expression_ taking priority over any option values of the _operand_.
+
 > [!NOTE]
 > The names of _options_ and their _values_ were derived from the
 > [options](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Intl/DateTimeFormat/resolvedOptions#description)
@@ -692,7 +697,15 @@ are encouraged to track development of these options during Tech Preview:
    - valid [Unicode Number System Identifier](https://cldr-smoke.unicode.org/spec/main/ldml/tr35.html#UnicodeNumberSystemIdentifier)
 - `timeZone` (default is system default time zone or UTC)
   - valid identifier per [BCP175](https://www.rfc-editor.org/rfc/rfc6557)
- 
+
+#### Composition
+
+When an _operand_ or an _option_ value uses a _variable_ annotated,
+directly or indirectly, by a `:datetime` _annotation_,
+its _resolved value_ contains an implementation-defined date/time value
+of the _operand_ of the annotated _expression_,
+together with the resolved options values.
+
 ### The `:date` function
 
 The function `:date` is used to format the date portion of date/time values.
@@ -716,6 +729,19 @@ The function `:date` has these _options_:
   - `medium` (default)
   - `short`
 
+If the _operand_ of the _expression_ is an implementation-defined date/time type,
+it can include other option values.
+Any _operand_ option values matching the `:datetime` _style options_ or _field options_ are ignored,
+as is any `style` option.
+
+#### Composition
+
+When an _operand_ or an _option_ value uses a _variable_ annotated,
+directly or indirectly, by a `:date` _annotation_,
+its _resolved value_ is implementation-defined.
+An implementation MAY emit a _Bad Operand_ or _Bad Option_ error (as appropriate)
+when this happens.
+
 ### The `:time` function
 
 The function `:time` is used to format the time portion of date/time values.
@@ -739,6 +765,18 @@ The function `:time` has these _options_:
   - `medium`
   - `short` (default)
 
+If the _operand_ of the _expression_ is an implementation-defined date/time type,
+it can include other option values.
+Any _operand_ option values matching the `:datetime` _style options_ or _field options_ are ignored,
+as is any `style` option.
+
+#### Composition
+
+When an _operand_ or an _option_ value uses a _variable_ annotated,
+directly or indirectly, by a `:time` _annotation_,
+its _resolved value_ is implementation-defined.
+An implementation MAY emit a _Bad Operand_ or _Bad Option_ error (as appropriate)
+when this happens.
 
 ### Date and Time Operands
 
@@ -788,5 +826,3 @@ For more information, see [Working with Timezones](https://w3c.github.io/timezon
 > The form of these serializations is known and is a de facto standard.
 > Support for these extensions is expected to be required in the post-tech preview.
 > See: https://datatracker.ietf.org/doc/draft-ietf-sedate-datetime-extended/
-
-

test/tests/functions/date.json

@@ -35,10 +35,10 @@
       "src": "{|2006-01-02| :date style=long}"
     },
     {
-      "src": ".local $d = {|2006-01-02| :date style=long} {{{$d :date}}}"
+      "src": ".local $d = {|2006-01-02| :date style=long} {{{$d}}}"
     },
     {
-      "src": ".local $t = {|2006-01-02T15:04:06| :time} {{{$t :date}}}"
+      "src": ".local $d = {|2006-01-02| :datetime dateStyle=long timeStyle=long} {{{$d :date}}}"
     }
   ]
 }

test/tests/functions/time.json

@@ -32,10 +32,10 @@
       "src": "{|2006-01-02T15:04:06| :time style=medium}"
     },
     {
-      "src": ".local $t = {|2006-01-02T15:04:06| :time style=medium} {{{$t :time}}}"
+      "src": ".local $t = {|2006-01-02T15:04:06| :time style=medium} {{{$t}}}"
     },
     {
-      "src": ".local $d = {|2006-01-02T15:04:06| :date} {{{$d :time}}}"
+      "src": ".local $t = {|2006-01-02T15:04:06| :datetime dateStyle=long timeStyle=long} {{{$t :time}}}"
     }
   ]
 }