Remove reflect refinement form nested quotes

This refinement is unsound as the nested `reflect` is not the same as the outer one.

In theory we could make it work by using the refinement
```
type Nested = Quotes {
  type Tree >: self.reflect.Tree
  ...
}
```
This refinement is unfortunately too large for the compiler to handle.

Removing the refinement forces us to transform trees into `Expr` before using them in a splice. This is in general a better practice anyway.

Author: nicolasstucki

library/src/scala/quoted/Quotes.scala

@@ -3697,21 +3697,7 @@ trait Quotes { self: runtime.QuoteUnpickler & runtime.QuoteMatching =>
 
   }
 
-  /** Type of a Quotes provided by a splice within a quote that took this context.
-   *  It is only required if working with the reflection API.
-   *
-   *  Usually it is infered by the quotes an splices typing. But sometimes it is necessary
-   *  to explicitly state that a context is nested as in the following example:
-   *
-   *  ```scala
-   *  def run(using Quotes)(tree: qctx.reflect.Tree): Unit =
-   *    def nested()(using qctx.Nested): Expr[Int] = '{  ${ makeExpr(tree) } + 1  }
-   *    '{  ${ nested() } + 2 }
-   *  def makeExpr(using Quotes)(tree: qctx.reflect.Tree): Expr[Int] = ???
-   *  ```
-   */
-  type Nested = Quotes {
-    val reflect: self.reflect.type
-  }
+  /** Type of a `Quotes` provided by a splice within a quote that took this context. */
+  type Nested = Quotes
 
 }

tests/neg-macros/i8045.scala

@@ -0,0 +1,5 @@
+import scala.quoted._
+object Test
+  def run(using Quotes)(tree: qctx.reflect.Tree): Unit =
+    '{  ${ makeExpr(tree) } + 1  } // error
+  def makeExpr(using Quotes)(tree: qctx.reflect.Tree): Expr[Int] = ???

tests/neg-macros/i8045b.scala

@@ -0,0 +1,8 @@
+import scala.quoted._
+object Test
+  def run(using q: Quotes)(tree: q.reflect.Tree): Unit =
+    def nested()(using q.Nested): Expr[Int] =
+      '{  ${ makeExpr(tree) } + 1  } // error
+    '{  ${ nested() } + 2 }
+
+  def makeExpr(using q: Quotes)(tree: q.reflect.Tree): Expr[Int] = ???

tests/pos-macros/i8045.scala

@@ -1,5 +1,5 @@
 import scala.quoted._
 object Test
   def run(using Quotes)(tree: quotes.reflect.Tree): Unit =
+    def makeExpr(tree: quotes.reflect.Tree): Expr[Int] = ???
     '{  ${ makeExpr(tree) } + 1  }
-  def makeExpr(using Quotes)(tree: quotes.reflect.Tree): Expr[Int] = ???

tests/pos-macros/i8045b.scala

@@ -1,8 +1,8 @@
 import scala.quoted._
 object Test
   def run(using q: Quotes)(tree: q.reflect.Tree): Unit =
+    def makeExpr(tree: q.reflect.Tree): Expr[Int] = ???
     def nested()(using q.Nested): Expr[Int] =
       '{  ${ makeExpr(tree) } + 1  }
     '{  ${ nested() } + 2 }
 
-  def makeExpr(using q: Quotes)(tree: q.reflect.Tree): Expr[Int] = ???

tests/run-staging/multi-staging.check

@@ -1,5 +1,5 @@
 stage1 code: ((q1: scala.quoted.Quotes) ?=> {
   val x1: scala.Int = 2
-  scala.quoted.runtime.Expr.quote[scala.Int](1.+(scala.quoted.runtime.Expr.nestedSplice[scala.Int](q1)(((evidence$5: q1.Nested) ?=> scala.quoted.Expr.apply[scala.Int](x1)(scala.quoted.Liftable.IntLiftable[scala.Int])(evidence$5))))).apply(using q1)
+  scala.quoted.runtime.Expr.quote[scala.Int](1.+(scala.quoted.runtime.Expr.nestedSplice[scala.Int](q1)(((evidence$5: scala.quoted.Quotes) ?=> scala.quoted.Expr.apply[scala.Int](x1)(scala.quoted.Liftable.IntLiftable[scala.Int])(evidence$5))))).apply(using q1)
 })
 3