Derived members succeed other givens in cycle test

In avoiding cycles in givens, assume quasi-synthetic derived
members come after other sibling givens (which are eligible).

This preserves the position of the derived symbol (which is not
synthetic); the corresponding tree is appended to the body of
the enclosing template.

The "derived" name prefix is conventional.

Author: som-snytt

compiler/src/dotty/tools/dotc/typer/Implicits.scala

@@ -1706,7 +1706,9 @@ trait Implicits:
           def candSucceedsGiven(sym: Symbol): Boolean =
             val owner = sym.owner
             if owner == candSym.owner then
-              sym.is(GivenVal) && sym.span.exists && sym.span.start <= candSym.span.start
+              sym.is(GivenVal)
+              && !sym.name.startsWith("derived$") // logically a synthetic injected at the end of the body
+              && sym.span.exists && sym.span.start <= candSym.span.start
             else if owner.isClass then false
             else candSucceedsGiven(owner)
 

tests/pos/i23897.scala

@@ -0,0 +1,11 @@
+
+import scala.deriving.Mirror
+
+class Test[A]
+object Test:
+  def derived[A](using m: Mirror.Of[A], t: Test[Int]): Test[A] = new Test[A]
+
+case class Foo(i: Int) derives Test
+object Foo:
+  //given i: Test[Int] = new Test[Int]
+  given Test[Int]()