Normative: Improve handling of grandfathered/private use tags

- Options which can't be applied to private use or grandfathered tags,
  whether script, region or Unicode extensions, trigger a RangeError
  in the constructor. (Closes #25 and #12)
- Insert additional calls to CanonicalizeLanguageTag for grandfathered
  tags, to enable options to be applied. (Closes #17)
- For grandfathered or private use tags, the entire tag is treated
  as the "language", both for the purposes of the
  Intl.Locale.prototype.language getter and for the language property
  of the options bag in the constructor.
  (Follows #25 (comment))

spec.html

@@ -34,10 +34,19 @@ <h1>ApplyOptionsToTag( _tag_, _options_ )</h1>
         1. Let _region_ be ? GetOption(_options_, `"region"`, `"string"`, *undefined*, *undefined*).
         1. If _region_ is not *undefined*, then
           1. If _region_ does not match the `region` production, throw a *RangeError* exception.
-        1. If _tag_ matches neither the `privateuse` nor the `grandfathered` production, then
-          1. Assert: _tag_ matches the `langtag` production.
-          1. If _language_ is not *undefined*, then
+        1. If _tag_ matches the `grandfathered` production,
+          1. Set _tag_ to CanonicalizeLanguageTag(_tag_).
+        1. If _language_ is not *undefined*,
+          1. If _tag_ matches the `privateuse` or `grandfathered` production,
+            1. Set _tag_ to _language_.
+            1. If _tag_ matches the `grandfathered` production,
+              1. Set _tag_ to CanonicalizeLanguageTag(_tag_).
+          1. Else,
+            1. Assert: _tag_ matches the `langtag` production.
             1. Set _tag_ to _tag_ with the substring corresponding to the `language` production replaced by the string _language_.
+        1. If _tag_ matches the `privateuse` or `grandfathered` production,
+          1. If _script_ is not *undefined*, or if _region_ is not *undefined*, throw a *RangeError* exception.
+        1. Else,
           1. If _script_ is not *undefined*, then
             1. If _tag_ does not contain a `script` production, then
               1. Set _tag_ to the concatenation of the `language` production of _tag_, `"-"`, _script_, and the rest of _tag_.
@@ -96,7 +105,7 @@ <h1>ApplyUnicodeExtensionToTag( _tag_, _options_, _relevantExtensionKeys_ )</h1>
         1. Let _locale_ be the String value that is _tag_ with all Unicode locale extension sequences removed.
         1. Let _newExtension_ be the canonicalized Unicode BCP 47 U Extension based on _attributes_ and _keywords_ as defined in <a href="https://www.unicode.org/reports/tr35/#u_Extension">UTS #35 section 3.6</a>.
         1. If _newExtension_ is not the empty String, then
-          1. Let _locale_ be ! InsertUnicodeExtension(_locale_, _newExtension_).
+          1. Let _locale_ be ? InsertUnicodeExtension(_locale_, _newExtension_).
         1. Set _result_.[[locale]] to _locale_.
         1. Return _result_.
       </emu-alg>
@@ -146,10 +155,14 @@ <h1>InsertUnicodeExtension( _locale_, _extension_ )</h1>
       <p>
         The InsertUnicodeExtension abstract operation inserts _extension_, which must be a Unicode locale extension sequence, into _locale_, which must be a String value with a structurally valid and canonicalized BCP 47 language tag. The following steps are taken:
       </p>
+      <p>
+        The following algorithm refers to <a href="https://tools.ietf.org/html/rfc5646#section-2.1">RFC 5646's Language-Tag grammar</a>.
+      </p>
 
       <emu-alg>
         1. Assert: _locale_ does not contain a substring that is a Unicode locale extension sequence.
         1. Assert: _extension_ is a Unicode extension sequence.
+        1. If _locale_ matches the `privateuse` or the `grandfathered` production, throw a *RangeError* exception.
         1. Let _privateIndex_ be ! Call(%StringProto_indexOf%, _locale_, &laquo; `"-x-"` &raquo;).
         1. If _privateIndex_ = -1, then
           1. Let _locale_ be the concatenation of _locale_ and _extension_.
@@ -402,7 +415,7 @@ <h1>get Intl.Locale.prototype.language</h1>
         1. If Type(_loc_) is not Object or _loc_ does not have an [[InitializedLocale]] internal slot, then
           1. Throw a *TypeError* exception.
         1. Let _locale_ be _loc_.[[Locale]].
-        1. If _locale_ matches the `privateuse` or the `grandfathered` production, return *undefined*.
+        1. If _locale_ matches the `privateuse` or the `grandfathered` production, return _locale_.
         1. Assert: _locale_ matches the `langtag` production.
         1. Return the substring of _locale_ corresponding to the `language` production.
       </emu-alg>