Give better feedback for classloader failure in staging

Author: nicolasstucki

staging/src/scala/quoted/staging/QuoteDriver.scala

@@ -53,7 +53,19 @@ private class QuoteDriver(appClassloader: ClassLoader) extends Driver:
         val method = clazz.getMethod("apply")
         val inst = clazz.getConstructor().newInstance()
 
-        method.invoke(inst).asInstanceOf[T]
+        try method.invoke(inst).asInstanceOf[T]
+        catch case ex: java.lang.reflect.InvocationTargetException =>
+          ex.getCause match
+            case ex: java.lang.NoClassDefFoundError =>
+              throw new Exception(
+                s"""`scala.quoted.staging.run` failed to load a class.
+                   |The classloader used for the `staging.Compiler` instance might not be the correct one.
+                   |Make sure that this classloader is the one that loaded the missing class.
+                   |Note that the classloader that loads the standard library might not be the same as
+                   |the one that loaded the application classes.""".stripMargin,
+                ex)
+
+            case _ => throw ex
     end match
 
   end run

tests/run-staging/i19170b.scala

@@ -0,0 +1,16 @@
+import scala.quoted.*
+
+given staging.Compiler =
+  staging.Compiler.make(getClass.getClassLoader) // warn: Suspicious top-level unqualified call to getClass
+
+class A(i: Int)
+
+def f(i: Expr[Int])(using Quotes): Expr[A] = { '{ new A($i) } }
+
+@main def Test = {
+  try
+    val g: Int => A = staging.run { '{ (i: Int) => ${ f('{i}) } } }
+    println(g(3))
+  catch case ex: Exception =>
+    assert(ex.getMessage().startsWith("`scala.quoted.staging.run` failed to load a class."))
+}