Restrict non-local private fields

Non-local private fields can be accessed by other instances of the class.
However, this creates the following issue:

```
inline trait A:
  private val x = 1
  def eq(o: A) = o.x == x

class B extends A
```

In the code above, when we inline the code into B, `o.x` accesses a private field of A from B, which is not allowed.
To allow this, we'd need to either make x protected, or change the signature of `eq` to take a B instead.

Author: timotheeandres

compiler/src/dotty/tools/dotc/inlines/Inlines.scala

@@ -639,11 +639,11 @@ object Inlines:
     private def inlinedMemberSym(sym: Symbol, withoutFlags: FlagSet = EmptyFlags)(using Context): Symbol =
       var name = sym.name
       var flags = sym.flags | Synthetic
-      if sym.isType || !sym.is(Private) then flags |= Override
-      if sym.isTermParamAccessor then
-        flags &~= ParamAccessor
-        if sym.is(Local) && sym.owner.isInlineTrait then
-          name = paramAccessorsMapper.registerNewName(sym)
+      if sym.isTermParamAccessor then flags &~= ParamAccessor
+      if sym.is(Local) then
+        name = paramAccessorsMapper.registerNewName(sym)
+      else
+        flags |= Override
       sym.copy(
         owner = ctx.owner,
         name = name,

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

@@ -155,6 +155,7 @@ class PostTyper extends MacroTransform with IdentityDenotTransformer { thisPhase
       tree match
         case tree: ValOrDefDef if !sym.is(Synthetic) =>
           checkInferredWellFormed(tree.tpt)
+          if tree.symbol.owner.isInlineTrait then checkInlTraitPrivateMemberIsLocal(tree)
           if sym.is(Method) then
             if sym.isSetter then
               sym.keepAnnotationsCarrying(thisPhase, Set(defn.SetterMetaAnnot))
@@ -192,6 +193,9 @@ class PostTyper extends MacroTransform with IdentityDenotTransformer { thisPhase
         => Checking.checkAppliedTypesIn(tree)
       case _ =>
 
+    private def checkInlTraitPrivateMemberIsLocal(tree: Tree)(using Context): Unit =
+      if tree.symbol.owner.isInlineTrait && tree.symbol.isAllOf(Private, butNot = Local) then
+        report.error(em"implementation restriction: inline traits cannot have non-local private members", tree.srcPos)
 
     private def transformSelect(tree: Select, targs: List[Tree])(using Context): Tree = {
       val qual = tree.qualifier

tests/neg/inline-trait-body-private-name-collision.scala

@@ -0,0 +1,3 @@
+inline trait A:
+  private val x: Int = 1 // error
+  def eq(o: A) = o.x == x