Add a section about tracked members to the modularity doc and update the

syntax reference

Author: KacperFKorban

compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -3489,7 +3489,7 @@ object Parsers {
      *  UsingClsTermParamClause::= ‘(’ ‘using’ [‘erased’] (ClsParams | ContextTypes) ‘)’
      *  ClsParams         ::=  ClsParam {‘,’ ClsParam}
      *  ClsParam          ::=  {Annotation}
-     *                         [{Modifier | ‘tracked’} (‘val’ | ‘var’)] Param
+     *                         [{Modifier} (‘val’ | ‘var’)] Param
      *  TypelessClause    ::= DefTermParamClause
      *                      | UsingParamClause
      *
@@ -3527,8 +3527,6 @@ object Parsers {
         if isErasedKw then
           mods = addModifier(mods)
         if paramOwner.isClass then
-          if isIdent(nme.tracked) && in.featureEnabled(Feature.modularity) && !in.lookahead.isColon then
-            mods = addModifier(mods)
           mods = addFlag(modifiers(start = mods), ParamAccessor)
           mods =
             if in.token == VAL then

docs/_docs/internals/syntax.md

@@ -141,7 +141,7 @@ type      val       var       while     with      yield
 ### Soft keywords
 
 ```
-as  derives  end  erased  extension  infix  inline  opaque  open  throws transparent  using  |  *  +  -
+as  derives  end  erased  extension  infix  inline  opaque  open  throws tracked transparent  using  |  *  +  -
 ```
 
 See the [separate section on soft keywords](../reference/soft-modifier.md) for additional
@@ -381,7 +381,7 @@ ClsParamClause    ::=  [nl] ‘(’ ClsParams ‘)’
                     |  [nl] ‘(’ ‘using’ (ClsParams | FunArgTypes) ‘)’
 ClsParams         ::=  ClsParam {‘,’ ClsParam}
 ClsParam          ::=  {Annotation}                                             ValDef(mods, id, tpe, expr) -- point of mods on val/var
-                       [{Modifier | ‘tracked’} (‘val’ | ‘var’)] Param
+                       [{Modifier} (‘val’ | ‘var’)] Param
 
 DefParamClauses   ::=  DefParamClause { DefParamClause } -- and two DefTypeParamClause cannot be adjacent
 DefParamClause    ::=  DefTypeParamClause
@@ -418,6 +418,7 @@ LocalModifier     ::=  ‘abstract’
                     |  ‘transparent’
                     |  ‘infix’
                     |  ‘erased’
+                    |  ‘tracked’
 
 AccessModifier    ::=  (‘private’ | ‘protected’) [AccessQualifier]
 AccessQualifier   ::=  ‘[’ id ‘]’

docs/_docs/reference/experimental/modularity.md

@@ -108,14 +108,6 @@ This works as it should now. Without the addition of `tracked` to the
 parameter of `SetFunctor` typechecking would immediately lose track of
 the element type `T` after an `add`, and would therefore fail.
 
-**Syntax Change**
-
-```
-ClsParam  ::=  {Annotation} [{Modifier | ‘tracked’} (‘val’ | ‘var’)] Param
-```
-
-The (soft) `tracked` modifier is only allowed for `val` parameters of classes.
-
 **Discussion**
 
 Since `tracked` is so useful, why not assume it by default? First, `tracked` makes sense only for `val` parameters. If a class parameter is not also a field declared using `val` then there's nothing to refine in the constructor result type. One could think of at least making all `val` parameters tracked by default, but that would be a backwards incompatible change. For instance, the following code would break:
@@ -134,6 +126,38 @@ only if the class refers to a type member of `x`. But it turns out that this
 scheme is unimplementable since it would quickly lead to cyclic references
 when typechecking recursive class graphs. So an explicit `tracked` looks like the best available option.
 
+## Tracked members
+
+The `tracked` modifier can also be used for `val` members of classes and traits
+to force the type of the member (or it's overriding member) to be as exact as
+possible. More precisely, it will will assign the `tracked` member the infered
+type of the rhs. For instance, consider the following definition:
+
+```scala
+trait F:
+  tracked val a: Int
+  tracked val b: Int
+
+class N extends F:
+  val a = 22 // a.type =:= 22
+  val b: Int = 22 // b.type =:= Int
+  tracked val c = 22 // c.type =:= 22
+```
+
+Here, the `tracked` modifier ensures that the type of `a` in `N` is `22` and not
+`Int`. But the type of `b` is `N` is `Int` since it's explicitly declared as
+`Int`. `tracked` members can also be immediately initialized, as in the case of
+`c`.
+
+## Tracked syntax change
+
+```
+LocalModifier     ::=  ‘tracked’
+```
+
+The (soft) `tracked` modifier is allowed as a local modifier.
+
+
 ## Allow Class Parents to be Refined Types
 
 Since `tracked` parameters create refinements in constructor types,

tests/neg/abstract-tracked-1.scala

@@ -0,0 +1,12 @@
+import scala.language.experimental.modularity
+import scala.language.future
+
+trait F:
+  tracked val a: Int
+
+class G:
+  val a: Int = 1
+
+def Test =
+  val g = new G
+  summon[g.a.type <:< 1] // error

tests/pos/abstract-tracked.scala

@@ -22,6 +22,9 @@ class L
 trait M:
   val f: Int
 
+class N extends F:
+  val a = 10
+
 object Test:
   val f = new F:
     val a = 1
@@ -39,6 +42,7 @@ object Test:
   }
   val m = new M:
     tracked val f = 9
+  val n = new N
 
   summon[f.a.type <:< 1]
   summon[g.b.type <:< 2]
@@ -48,3 +52,4 @@ object Test:
   summon[k.d.type <:< 7]
   // summon[l.e.type <:< 8] // unrelated issue -- error: e is not a member of L
   summon[m.f.type <:< 9]
+  summon[n.a.type <:< 10]