Allow normal parameters after given parameters

With the new given syntax, we can lift the restriction that normal
parameters cannot follow given parameters. This restriction was introduced
because with the previous syntax this looked too awkward.

The added test `given-eta` shows that eta expansion works as expected for these cases.

Author: odersky

compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -2849,7 +2849,7 @@ object Parsers {
                      ofCaseClass: Boolean = false,
                      ofInstance: Boolean = false): List[List[ValDef]] =
 
-      def recur(firstClause: Boolean, nparams: Int, contextualOnly: Boolean): List[List[ValDef]] =
+      def recur(firstClause: Boolean, nparams: Int): List[List[ValDef]] =
         newLineOptWhenFollowedBy(LPAREN)
         if in.token == LPAREN then
           val paramsStart = in.offset
@@ -2858,23 +2858,14 @@ object Parsers {
               ofClass = ofClass,
               ofCaseClass = ofCaseClass,
               firstClause = firstClause)
-          val isGiven = params match
-            case param :: _ if param.mods.flags.is(Given) => true
-            case _ =>
-              if contextualOnly then
-                syntaxError(
-                  if ofInstance then em"parameters of instance definitions must be `given' clauses"
-                  else em"normal parameters cannot come after `given' clauses",
-                  paramsStart)
-              false
           val lastClause = params.nonEmpty && params.head.mods.flags.is(Implicit)
           params :: (
             if lastClause then Nil
-            else recur(firstClause = false, nparams + params.length, isGiven))
+            else recur(firstClause = false, nparams + params.length))
         else Nil
       end recur
 
-      recur(firstClause = true, 0, ofInstance)
+      recur(firstClause = true, 0)
     end paramClauses
 
 /* -------- DEFS ------------------------------------------- */

docs/docs/reference/contextual-new/given-clauses.md

@@ -69,7 +69,7 @@ maximum(xs)(given descending(given ListOrd(given IntOrd)))
 There can be several `given` parameter clauses in a definition and `given` parameter clauses can be freely
 mixed with normal ones. Example:
 ```scala
-def f(u: Universe)(given c: u.Context)(given s: ctx.Symbol, k: ctx.Kind) = ...
+def f(u: Universe)(given ctx: u.Context)(given s: ctx.Symbol, k: ctx.Kind) = ...
 ```
 Multiple given clauses are matched left-to-right in applications. Example:
 ```scala