Harden canAssign (#15963)

Fixes #15897

Author: KacperFKorban

compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -989,7 +989,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
 
         def canAssign(sym: Symbol) =
           sym.is(Mutable, butNot = Accessor) ||
-          ctx.owner.isPrimaryConstructor && !sym.is(Method) && sym.owner == ctx.owner.owner ||
+          ctx.owner.isPrimaryConstructor && !sym.is(Method) && sym.maybeOwner == ctx.owner.owner ||
             // allow assignments from the primary constructor to class fields
           ctx.owner.name.is(TraitSetterName) || ctx.owner.isStaticConstructor
 

tests/neg/i15897.scala

@@ -0,0 +1,8 @@
+object O {
+  class AC(code: => Unit)
+
+  val action = new AC({mode = ???}) {} // error
+
+  def mode: AnyRef = ???
+  def mode=(em: AnyRef): Unit = {} // error // error // error
+}

tests/pos/i15897.scala

@@ -0,0 +1,8 @@
+object O {
+  class AC(code: => Unit)
+
+  val action = new AC({mode = ???}) {}
+
+  def mode: AnyRef = ???
+  def mode_=(em: AnyRef): Unit = {}
+}