Move covariant keyword docs out of sound problems page (#6299)

Contributes to #6265

firebase.json

@@ -266,7 +266,7 @@
       { "source": "/keyword/class", "destination": "/language/classes#instance-variables", "type": 301 },
       { "source": "/keyword/const", "destination": "/language/variables#final-and-const", "type": 301 },
       { "source": "/keyword/continue", "destination": "/language/loops#break-and-continue", "type": 301 },
-      { "source": "/keyword/covariant", "destination": "/deprecated/sound-problems#the-covariant-keyword", "type": 301 },
+      { "source": "/keyword/covariant", "destination": "/language/type-system#covariant-keyword", "type": 301 },
       { "source": "/keyword/default", "destination": "/language/branches#switch", "type": 301 },
       { "source": "/keyword/deferred", "destination": "/language/libraries#lazily-loading-a-library", "type": 301 },
       { "source": "/keyword/do", "destination": "/language/loops#while-and-do-while", "type": 301 },

src/_data/glossary.yml

@@ -614,7 +614,7 @@
     - text: "Covariance and contravariance"
       link: "https://en.wikipedia.org/wiki/Covariance_and_contravariance_(computer_science)"
     - text: "The covariant keyword"
-      link: "/deprecated/sound-problems#the-covariant-keyword"
+      link: "/language/type-system#covariant-keyword"
   labels:
     - "language"
     - "type system"

src/_data/keywords.yml

@@ -35,7 +35,7 @@
   link: /language/loops#break-and-continue
   type: reserved
 - term: 'covariant'
-  link: /deprecated/sound-problems#the-covariant-keyword
+  link: /language/type-system#covariant-keyword
   type: bit
 - term: 'default'
   link: /language/branches#switch

src/content/deprecated/sound-problems.md

@@ -257,7 +257,7 @@ For more information, see
 
 :::note
 If you have a valid reason to use a subtype, you can use the
-[covariant keyword](#the-covariant-keyword).
+[covariant keyword](/language/type-system#covariant-keyword).
 :::
 
 <hr>
@@ -600,35 +600,8 @@ assumeStrings(names.[!cast!]<String>());
 
 ### The covariant keyword
 
-Some (rarely used) coding patterns rely on tightening a type
-by overriding a parameter's type with a subtype, which is invalid.
-In this case, you can use the `covariant` keyword to
-tell the analyzer that you are doing this intentionally.
-This removes the static error and instead checks for an invalid
-argument type at runtime.
-
-The following shows how you might use `covariant`:
-
-<?code-excerpt "lib/covariant.dart" replace="/covariant/[!$&!]/g"?>
-```dart tag=passes-sa
-class Animal {
-  void chase(Animal x) { ... }
-}
-
-class Mouse extends Animal { ... }
-
-class Cat extends Animal {
-  @override
-  void chase([!covariant!] Mouse x) { ... }
-}
-```
-
-Although this example shows using `covariant` in the subtype,
-the `covariant` keyword can be placed in either the superclass
-or the subclass method.
-Usually the superclass method is the best place to put it.
-The `covariant` keyword applies to a single parameter and is
-also supported on setters and fields.
+The documentation on the `covariant` keyword has been
+moved to [The Dart Type system](/language/type-system#covariant-keyword).
 
 [bottom type]: https://en.wikipedia.org/wiki/Bottom_type
 [cast()]: {{site.dart-api}}/dart-core/Iterable/cast.html

src/content/language/extend.md

@@ -78,7 +78,7 @@ it's similar to a downcast in that it can cause a type error at runtime.
 Still, narrowing the type is possible
 if the code can guarantee that a type error won't occur.
 In this case, you can use the
-[`covariant` keyword](/deprecated/sound-problems#the-covariant-keyword)
+[`covariant` keyword](/language/type-system#covariant-keyword)
 in a parameter declaration.
 For details, see the
 [Dart language specification][].

src/content/language/type-system.md

@@ -179,7 +179,7 @@ subtype of the original parameter.
 
 :::note
 If you have a valid reason to use a subtype, you can use the
-[`covariant` keyword](/deprecated/sound-problems#the-covariant-keyword).
+[`covariant` keyword](/language/type-system#covariant-keyword).
 :::
 
 Consider the `chase(Animal)` method for the `Animal` class:
@@ -483,6 +483,38 @@ For more information, see
 [Use sound return types when overriding methods](#use-proper-return-types)
 and [Use sound parameter types when overriding methods](#use-proper-param-types).
 
+<a id="covariant-keyword" aria-hidden="true"></a>
+#### Covariant parameters
+
+Some (rarely used) coding patterns rely on tightening a type
+by overriding a parameter's type with a subtype, which is invalid.
+In this case, you can use the `covariant` keyword to
+tell the analyzer that you're doing this intentionally.
+This removes the static error and instead checks for an invalid
+argument type at runtime.
+
+The following shows how you might use `covariant`:
+
+<?code-excerpt "lib/covariant.dart" replace="/covariant/[!$&!]/g"?>
+```dart tag=passes-sa
+class Animal {
+  void chase(Animal x) { ... }
+}
+
+class Mouse extends Animal { ... }
+
+class Cat extends Animal {
+  @override
+  void chase([!covariant!] Mouse x) { ... }
+}
+```
+
+Although this example shows using `covariant` in the subtype,
+the `covariant` keyword can be placed in either the superclass
+or the subclass method.
+Usually the superclass method is the best place to put it.
+The `covariant` keyword applies to a single parameter and is
+also supported on setters and fields.
 
 ## Other resources