repl: Use the correct parent Classloader on Java 9+

On Java 9+, `null` as a parent of a URLClassLoader means the parent is
the bootstrap classloader which doesn't contain modules like `java.sql`,
explicitly use the platform classloader instead.

Fixes #11646.

Author: smarter

compiler/src/dotty/tools/repl/Rendering.scala

@@ -47,7 +47,14 @@ private[repl] class Rendering(parentClassLoader: Option[ClassLoader] = None) {
     else {
       val parent = parentClassLoader.getOrElse {
         val compilerClasspath = ctx.platform.classPath(using ctx).asURLs
-        new java.net.URLClassLoader(compilerClasspath.toArray, null)
+        // We can't use the system classloader as a parent because it would
+        // pollute the user classpath with everything passed to the JVM
+        // `-classpath`. We can't use `null` as a parent either because on Java
+        // 9+ that's the bootstrap classloader which doesn't contain modules
+        // like `java.sql`, so we use the parent of the system classloader,
+        // which should correspond to the platform classloader on Java 9+.
+        val baseClassLoader = ClassLoader.getSystemClassLoader.getParent
+        new java.net.URLClassLoader(compilerClasspath.toArray, baseClassLoader)
       }
 
       myClassLoader = new AbstractFileClassLoader(ctx.settings.outputDir.value, parent)

compiler/test-resources/repl/defaultClassloader

@@ -0,0 +1,3 @@
+scala> val d: java.sql.Date = new java.sql.Date(100L)
+val d: java.sql.Date = 1970-01-01
+