diff --git a/compiler/src/dotty/tools/dotc/config/PathResolver.scala b/compiler/src/dotty/tools/dotc/config/PathResolver.scala
index 29e6e35855c8..f60727e6bba2 100644
--- a/compiler/src/dotty/tools/dotc/config/PathResolver.scala
+++ b/compiler/src/dotty/tools/dotc/config/PathResolver.scala
@@ -36,9 +36,16 @@ object PathResolver {
   /** Values found solely by inspecting environment or property variables.
    */
   object Environment {
-    private def searchForBootClasspath = (
-      systemProperties find (_._1 endsWith ".boot.class.path") map (_._2) getOrElse ""
-    )
+    private def searchForBootClasspath = {
+      import scala.jdk.CollectionConverters.*
+      val props = System.getProperties
+      // This formulation should be immune to ConcurrentModificationExceptions when system properties
+      // we're unlucky enough to witness a partially published result of System.setProperty or direct
+      // mutation of the System property map. stringPropertyNames internally uses the Enumeration interface,
+      // rather than Iterator, and this disables the fail-fast ConcurrentModificationException.
+      val propNames = props.stringPropertyNames()
+      propNames.asScala collectFirst { case k if k endsWith ".boot.class.path" => props.getProperty(k) } getOrElse ""
+    }
 
     /** Environment variables which java pays attention to so it
      *  seems we do as well.
@@ -46,7 +53,8 @@ object PathResolver {
     def classPathEnv: String        =  envOrElse("CLASSPATH", "")
     def sourcePathEnv: String       =  envOrElse("SOURCEPATH", "")
 
-    def javaBootClassPath: String   = propOrElse("sun.boot.class.path", searchForBootClasspath)
+    //using propOrNone/getOrElse instead of propOrElse so that searchForBootClasspath is lazy evaluated
+    def javaBootClassPath: String   = propOrNone("sun.boot.class.path") getOrElse searchForBootClasspath
 
     def javaExtDirs: String         = propOrEmpty("java.ext.dirs")
     def scalaHome: String           = propOrEmpty("scala.home")
diff --git a/compiler/src/dotty/tools/dotc/typer/RefChecks.scala b/compiler/src/dotty/tools/dotc/typer/RefChecks.scala
index 0a0356707048..0ec9458cac5c 100644
--- a/compiler/src/dotty/tools/dotc/typer/RefChecks.scala
+++ b/compiler/src/dotty/tools/dotc/typer/RefChecks.scala
@@ -525,7 +525,6 @@ object RefChecks {
 
       // todo: align accessibility implication checking with isAccessible in Contexts
       def isOverrideAccessOK =
-        val memberIsPublic = (member.flags & AccessFlags).isEmpty && !member.privateWithin.exists
         def protectedOK = !other.is(Protected) || member.is(Protected)        // if o is protected, so is m
         def accessBoundaryOK =
           val ob = other.accessBoundary(member.owner)
@@ -534,7 +533,7 @@ object RefChecks {
           def companionBoundaryOK = ob.isClass && !ob.isLocalToBlock && mb.is(Module) && (ob.companionModule eq mb.companionModule)
           ob.isContainedIn(mb) || companionBoundaryOK    // m relaxes o's access boundary,
         def otherIsJavaProtected = other.isAllOf(JavaProtected)               // or o is Java defined and protected (see #3946)
-        memberIsPublic || protectedOK && (accessBoundaryOK || otherIsJavaProtected)
+        member.isPublic || protectedOK && (accessBoundaryOK || otherIsJavaProtected)
       end isOverrideAccessOK
 
       if !member.hasTargetName(other.targetName) then
@@ -1169,16 +1168,18 @@ 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)
+          member.symbol.isPublic && {
+            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 --git a/dist/libexec/cli-common-platform b/dist/libexec/cli-common-platform
index a5906e882bb4..e56f5221dbf2 100644
--- a/dist/libexec/cli-common-platform
+++ b/dist/libexec/cli-common-platform
@@ -1,3 +1,3 @@
 #!/usr/bin/env bash
 
-SCALA_CLI_CMD_BASH=("\"$JAVACMD\"" "-jar \"$PROG_HOME/bin/scala-cli.jar\"")
+SCALA_CLI_CMD_BASH=("\"$JAVACMD\"" "-jar \"$PROG_HOME/libexec/scala-cli.jar\"")
diff --git a/dist/libexec/cli-common-platform.bat b/dist/libexec/cli-common-platform.bat
index 99103266c1d9..45b09f3460e6 100644
--- a/dist/libexec/cli-common-platform.bat
+++ b/dist/libexec/cli-common-platform.bat
@@ -2,4 +2,4 @@
 
 @rem we need to escape % in the java command path, for some reason this doesnt work in common.bat
 set "_JAVACMD=!_JAVACMD:%%=%%%%!"
-set SCALA_CLI_CMD_WIN="%_JAVACMD%" "-jar" "%_PROG_HOME%\bin\scala-cli.jar"
\ No newline at end of file
+set SCALA_CLI_CMD_WIN="%_JAVACMD%" "-jar" "%_PROG_HOME%\libexec\scala-cli.jar"
diff --git a/tests/warn/i21816.scala b/tests/warn/i21816.scala
new file mode 100644
index 000000000000..9153b8b0ee2f
--- /dev/null
+++ b/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