ECMA-402 v1 constructor compatibility semantics for DateTimeFormat

spec/datetimeformat.html

@@ -296,6 +296,26 @@ <h1>ToLocalTime (date, calendar, timeZone)</h1>
         It is recommended that implementations use the time zone information of the IANA Time Zone Database.
       </emu-note>
     </emu-clause>
+
+    <emu-clause id="sec-unwrapdatetimeformat" aoid="UnwrapDateTimeFormat">
+      <h1>UnwrapDateTimeFormat(nf)</h1>
+      <p>
+        The UnwrapDateTimeFormat abstract operation gets the underlying DateTimeFormat operation
+        for various methods which implement ECMA-402 v1 semantics for supporting initializing
+        existing Intl objects.
+      </p>
+      <emu-alg>
+        1. If _nf_ does not have an [[initializedDateTImeFormat]] internal slot,
+          1. If ? InstanceofOperator(_nf_, %DateTimeFormat%),
+            1. Let _fallback_ be ? RequireObjectCoercible(Get(_nf_, Intl.[[FallbackSymbol]])).
+            1. If _fallback_ has an [[initializedDateTimeFormat]] internal slot,
+              1. Let _nf_ be _fallback_.
+            1. Else, throw a *TypeError* exception.
+          1. Else, throw a *TypeError* exception.
+        1. Return _nf_.
+      </emu-alg>
+    </emu-clause>
+  </emu-clause>
   </emu-clause>
 
   <emu-clause id="sec-intl-datetimeformat-constructor">
@@ -314,6 +334,9 @@ <h1>Intl.DateTimeFormat ([ locales [ , options ]])</h1>
 
       <emu-alg>
         1. If NewTarget is *undefined*, let _newTarget_ be the active function object, else let _newTarget_ be NewTarget.
+        1. If _this_ is not *undefined*, and ? InstanceofOperator(_this_, %DateTimeFormat%),
+          1. Perform ? DefineOwnPropertyOrThrow(_this_, Intl.[[FallbackSymbol]], { [[Value]]: Construct(%DateTimeFormat%, _locales_, _options_), [[Writable]]: *false*, [[Enumerable]]: *false*, [[Configurable]]: *false* }).
+          1. Return _this_.
         1. Let _dateTimeFormat_ be ? OrdinaryCreateFromConstructor(_newTarget_, `"%DateTimeFormatPrototype%"`, « [[initializedIntlObject]], [[initializedDateTimeFormat]], [[locale]], [[calendar]], [[numberingSystem]], [[timeZone]], [[weekday]], [[era]], [[year]], [[month]], [[day]], [[hour]], [[minute]], [[second]], [[timeZoneName]], [[hour12]], [[hourNo0]], [[pattern]], [[boundFormat]] »).
         1. Return ? InitializeDateTimeFormat(_dateTimeFormat_, _locales_, _options_).
       </emu-alg>
@@ -448,7 +471,7 @@ <h1>get Intl.DateTimeFormat.prototype.format</h1>
       <emu-alg>
         1. Let _dtf_ be *this* value.
         1. If Type(_dtf_) is not Object, throw a *TypeError* exception.
-        1. If _dtf_ does not have an [[initializedDateTimeFormat]] internal slot, throw a *TypeError* exception.
+        1. Let _dtf_ be ? UnwrapDateTimeFormat(_dtf_).
         1. If the [[boundFormat]] internal slot of _dtf_ is *undefined*, then
           1. Let _F_ be a new built-in function object as defined in DateTime Format Functions (<emu-xref href="#sec-datetime-format-functions"></emu-xref>).
           1. Let _bf_ be BoundFunctionCreate(_F_, _dft_, « »).
@@ -462,7 +485,7 @@ <h1>get Intl.DateTimeFormat.prototype.format</h1>
       <h1>Intl.DateTimeFormat.prototype.resolvedOptions ()</h1>
 
       <p>
-        This function provides access to the locale and formatting options computed during initialization of the object.
+        This function provides access to the locale and formatting options computed during initialization of the object. This function initially invokes the internal algorithm UnwrapDateTimeFormat to get the %DateTimeFormat% object on which to operate.
       </p>
 
       <p>