Fix trait setter validity

Trait setters had a validity interval that started before they were created.
This caused stale symbol errors in macro compilations.

Author: odersky

compiler/src/dotty/tools/dotc/transform/Mixin.scala

@@ -10,6 +10,7 @@ import SymUtils._
 import Symbols._
 import SymDenotations._
 import Types._
+import Periods._
 import Decorators._
 import DenotTransformers._
 import StdNames._
@@ -146,6 +147,7 @@ class Mixin extends MiniPhase with SymTransformer { thisPhase =>
         // !decl.isClass avoids forcing nested traits, preventing cycles
         if !decl.isClass && needsTraitSetter(decl) then
           val setter = makeTraitSetter(decl.asTerm)
+          setter.validFor = Period(ctx.runId, thisPhase.next.id, decl.validFor.lastPhaseId)
           decls1.enter(setter)
           modified = true
       if modified then

tests/pos/i12140/Test.scala

@@ -0,0 +1 @@
+@main def Test = println(example.Trait.get)

tests/pos/i12140/Trait.scala

@@ -0,0 +1,14 @@
+// Trait.scala
+package example
+
+import quoted._
+
+trait Trait {
+  implicit val foo: Int = 23
+}
+
+object Trait {
+  inline def get: Trait = ${ getImpl }
+
+  def getImpl(using Quotes): Expr[Trait] = '{ new Trait {} }
+}