Change name generation scheme for anonymous givens.

Instead of `T_given` it's now `given_T`.

Reason: we want to follow the convention that these names start with a lowercase letter.

Author: odersky

compiler/src/dotty/tools/dotc/ast/Desugar.scala

@@ -933,7 +933,7 @@ object desugar {
 
   /** Invent a name for an anonympus given of type or template `impl`. */
   def inventGivenName(impl: Tree)(implicit ctx: Context): SimpleName =
-    avoidIllegalChars(s"${inventName(impl)}_given".toTermName.asSimpleName)
+    avoidIllegalChars(s"given_${inventName(impl)}".toTermName.asSimpleName)
 
   /** The normalized name of `mdef`. This means
    *   1. Check that the name does not redefine a Scala core class.

compiler/test/dotty/tools/repl/ReplCompilerTests.scala

@@ -132,7 +132,7 @@ class ReplCompilerTests extends ReplTest {
     fromInitialState { implicit state => run("given as Int = 10") }
     .andThen         { implicit state =>
       assertEquals(
-        "def Int_given: Int",
+        "def given_Int: Int",
         storedOutput().trim
       )
       run("implicitly[Int]")

docs/docs/reference/contextual-new/relationship-implicits.md

@@ -43,7 +43,7 @@ Given instances can be mapped to combinations of implicit objects, classes and i
     would map to
     ```scala
       final implicit lazy val global: ExecutionContext = new ForkJoinContext()
-      final implicit def Context_given = ctx
+      final implicit def given_Context = ctx
     ```
 
 ### Anonymous Given Instances

tests/pos/reference/delegates.scala

@@ -226,13 +226,13 @@ object Implicits extends Common {
   }
   implicit def ListOrd[T: Ord]: Ord[List[T]] = new ListOrd[T]
 
-  class Convertible_List_List_given[From, To](implicit c: Convertible[From, To])
+  class given_Convertible_List_List[From, To](implicit c: Convertible[From, To])
   extends Convertible[List[From], List[To]] {
     def (x: List[From]) convert: List[To] = x.map(c.convert)
   }
-  implicit def Convertible_List_List_given[From, To](implicit c: Convertible[From, To])
+  implicit def given_Convertible_List_List[From, To](implicit c: Convertible[From, To])
     : Convertible[List[From], List[To]] =
-    new Convertible_List_List_given[From, To]
+    new given_Convertible_List_List[From, To]
 
   def maximum[T](xs: List[T])
                 (implicit cmp: Ord[T]): T =