Support def-local tracked members

Weaken the check for legal tracked members

compiler/src/dotty/tools/dotc/ast/Desugar.scala

@@ -1574,9 +1574,10 @@ object desugar {
   }
 
   val legalTracked: Context ?=> MemberDefTest = {
-    case valdef @ ValDef(_, _, _) =>
-      val sym = valdef.symbol
-      !ctx.owner.exists || ctx.owner.isClass || ctx.owner.is(Case) || ctx.owner.isConstructor || valdef.mods.is(Param) || valdef.mods.is(ParamAccessor)
+    case valdef: ValDef if valdef.mods.is(Param) || valdef.mods.is(ParamAccessor) =>
+      !ctx.owner.exists || ctx.owner.isClass || ctx.owner.is(Case) || ctx.owner.isConstructor || ctx.owner.is(Synthetic)
+    case valdef: ValDef =>
+      true
   }
 
   def checkOpaqueAlias(tree: MemberDef)(using Context): MemberDef =

tests/neg/abstract-tracked.check

@@ -14,7 +14,3 @@
 7 |  tracked def f: F // error
   |  ^^^^^^^^^^^^^^^^
   |  Modifier tracked is not allowed for this definition
--- [E156] Syntax Error: tests/neg/abstract-tracked.scala:14:14 ---------------------------------------------------------
-14 |  tracked val x = 1 // error
-   |  ^^^^^^^^^^^^^^^^^
-   |  Modifier tracked is not allowed for this definition

tests/neg/abstract-tracked.scala

@@ -9,6 +9,3 @@ trait G:
 tracked object O // error
 
 tracked class C // error
-
-def f =
-  tracked val x = 1 // error

tests/neg/tracked.check

@@ -14,10 +14,6 @@
 4 |class C2(tracked var x: Int) // error
   |                     ^
   |                     mutable variables may not be `tracked`
--- [E156] Syntax Error: tests/neg/tracked.scala:8:16 -------------------------------------------------------------------
-8 |    tracked val b: Int = 2 // error
-  |    ^^^^^^^^^^^^^^^^^^^^^^
-  |    Modifier tracked is not allowed for this definition
 -- [E156] Syntax Error: tests/neg/tracked.scala:11:17 ------------------------------------------------------------------
 11 |  tracked object Foo // error
    |  ^^^^^^^^^^^^^^^^^^

tests/neg/tracked.scala

@@ -5,7 +5,7 @@ class C2(tracked var x: Int) // error
 
 object A:
   def foo(tracked a: Int) = // error
-    tracked val b: Int = 2 // error
+    tracked val b: Int = 2
 
 object B:
   tracked object Foo // error

tests/pos/tracked-val-weaker.scala

@@ -0,0 +1,13 @@
+import scala.language.experimental.modularity
+
+object O:
+  tracked val x = 1
+
+def Test =
+  tracked val x = 1
+  val _: 1 = O.x
+  val _: 1 = x
+  tracked val y = x
+  val _: x.type = y
+  tracked val y1 = O.x
+  val _: O.x.type = y1