Nowarn extension matching nonpublic member

Author: som-snytt

Diff for: compiler/src/dotty/tools/dotc/typer/RefChecks.scala

@@ -1169,16 +1169,19 @@ object RefChecks {
         target.nonPrivateMember(sym.name)
         .filterWithPredicate:
           member =>
-          val memberIsImplicit = member.info.hasImplicitParams
-          val paramTps =
-            if memberIsImplicit then methTp.stripPoly.firstParamTypes
-            else methTp.firstExplicitParamTypes
-
-          paramTps.isEmpty || memberIsImplicit && !methTp.hasImplicitParams || {
-            val memberParamTps = member.info.stripPoly.firstParamTypes
-            !memberParamTps.isEmpty
-            && memberParamTps.lengthCompare(paramTps) == 0
-            && memberParamTps.lazyZip(paramTps).forall((m, x) => x frozen_<:< m)
+          val memberIsPublic = (member.symbol.flags & AccessFlags).isEmpty && !member.symbol.privateWithin.exists
+          memberIsPublic && {
+            val memberIsImplicit = member.info.hasImplicitParams
+            val paramTps =
+              if memberIsImplicit then methTp.stripPoly.firstParamTypes
+              else methTp.firstExplicitParamTypes
+
+            paramTps.isEmpty || memberIsImplicit && !methTp.hasImplicitParams || {
+              val memberParamTps = member.info.stripPoly.firstParamTypes
+              !memberParamTps.isEmpty
+              && memberParamTps.lengthCompare(paramTps) == 0
+              && memberParamTps.lazyZip(paramTps).forall((m, x) => x frozen_<:< m)
+            }
           }
         .exists
       if !target.typeSymbol.denot.isAliasType && !target.typeSymbol.denot.isOpaqueAlias && hidden

Diff for: tests/warn/i21816.scala

@@ -0,0 +1,17 @@
+
+case class CC(a: String, b: String) extends Iterable[String] {
+  override def iterator: Iterator[String] = Iterator(a, b)
+}
+
+trait T {
+  extension (cc: CC) def className: String = "foo"
+}
+
+object O extends T {
+  def foo = {
+    val cc = CC("a", "b")
+    println(cc.className)
+  }
+}
+
+@main def main() = O.foo