Also consider @TargetNAME when checking private overrides

Fixes #18244

Author: odersky

compiler/src/dotty/tools/dotc/core/SymDenotations.scala

@@ -1353,7 +1353,7 @@ object SymDenotations {
      *
      *                   site: Subtype of both inClass and C
      */
-    final def matchingDecl(inClass: Symbol, site: Type)(using Context): Symbol = {
+    final def matchingDecl(inClass: Symbol, site: Type, name: Name = this.name)(using Context): Symbol = {
       var denot = inClass.info.nonPrivateDecl(name)
       if (denot.isTerm) // types of the same name always match
         denot = denot.matchingDenotation(site, site.memberInfo(symbol), symbol.targetName)

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

@@ -982,7 +982,9 @@ object RefChecks {
     then
       val cls = sym.owner.asClass
       for bc <- cls.baseClasses.tail do
-        val other = sym.matchingDecl(bc, cls.thisType)
+        var other = sym.matchingDecl(bc, cls.thisType)
+        if !other.exists && sym.targetName != sym.name then
+          other = sym.matchingDecl(bc, cls.thisType, sym.targetName)
         if other.exists then
           report.error(em"private $sym cannot override ${other.showLocated}", sym.srcPos)
   end checkNoPrivateOverrides

tests/neg/i18244.scala

@@ -0,0 +1,12 @@
+import scala.annotation.*
+
+class A:
+  def foo: Int = 1
+class B extends A:
+  @targetName("foo") private[this] def bla: Int = 2 // error
+class C extends A:
+  @targetName("foo") private def bla: Int = 2 // error
+
+@main def Test =
+  val b = new B
+  println(b.foo)