Update staging.Compiler.make documentation

This addresses part of #19211, #19170, and #19176

Author: nicolasstucki

docs/_spec/TODOreference/metaprogramming/staging.md

@@ -108,7 +108,11 @@ to get a source-like representation of the expression.
 import scala.quoted.*
 
 // make available the necessary compiler for runtime code generation
-given staging.Compiler = staging.Compiler.make(getClass.getClassLoader)
+given staging.Compiler =
+  // We need an instance of a class that is defined in the current application (not the standard library)
+  // `this` can be used instead of an instance of `Dummy` if the Compiler is instantiated within one of the application classes.
+  object Dummy
+  staging.Compiler.make(Dummy.getClass.getClassLoader)
 
 val f: Array[Int] => Int = staging.run {
   val stagedSum: Expr[Array[Int] => Int] =

staging/src/scala/quoted/staging/Compiler.scala

@@ -13,15 +13,27 @@ object Compiler:
 
   /** Create a new instance of the compiler using the the classloader of the application.
    *
-   * Usage:
-   * ```
-   * import scala.quoted.staging._
-   * given Compiler = Compiler.make(getClass.getClassLoader)
-   * ```
+   *  Usage:
+   *  ```
+   *  import scala.quoted.staging._
+   *  given Compiler =
+   *    object Dummy
+   *    Compiler.make(Dummy.getClass.getClassLoader)
+   *  ```
    *
-   * @param appClassloader classloader of the application that generated the quotes
-   * @param settings compiler settings
-   * @return A new instance of the compiler
+   *  Note that we use an instance of `Dummy` to get the classloader that loaded the application.
+   *  Any other instance of a class defined in the application would also work.
+   *  Using a class defined in the standard library should be avoided as it might be loaded by a different classloader.
+   *
+   *  If the given compiler is defined in one of your classes (e.i. not as a top-level definition), then
+   *  the compiler can be instantiated with:
+   *  ```
+   *  given Compiler = Compiler.make(this.getClass.getClassLoader)
+   *  ```
+   *
+   *  @param appClassloader classloader of the application that generated the quotes
+   *  @param settings compiler settings
+   *  @return A new instance of the compiler
    */
   def make(appClassloader: ClassLoader)(implicit settings: Settings): Compiler =
     new Compiler:

tests/run-staging/i11162.scala

@@ -2,7 +2,7 @@ import scala.quoted.*
 
 object Test {
 
-  given staging.Compiler = staging.Compiler.make(getClass.getClassLoader)
+  given staging.Compiler = staging.Compiler.make(this.getClass.getClassLoader)
 
   def main(args: Array[String]): Unit =
     staging.run {

tests/run-staging/i19170.scala

@@ -0,0 +1,14 @@
+import scala.quoted.*
+
+given staging.Compiler =
+  object Dummy
+  staging.Compiler.make(Dummy.getClass.getClassLoader)
+
+class A(i: Int)
+
+def f(i: Expr[Int])(using Quotes): Expr[A] = { '{ new A($i) } }
+
+@main def Test = {
+  val g: Int => A = staging.run { '{ (i: Int) => ${ f('{i}) } } }
+  println(g(3))
+}

tests/run-staging/i19176.scala

@@ -0,0 +1,10 @@
+import scala.quoted.*
+
+given staging.Compiler =
+  object Dummy
+  staging.Compiler.make(Dummy.getClass.getClassLoader)
+
+class A
+val f: (A, Int) => Int = staging.run { '{ (q: A, x: Int) => x } }
+
+@main def Test = f(new A, 3)