Fix docs and Parser

Author: odersky

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

@@ -3121,12 +3121,12 @@ object Parsers {
         makeConstructor(Nil, vparamss, rhs).withMods(mods).setComment(in.getDocComment(start))
       }
       else {
-        def extParamss = try paramClause(0, prefix = true) :: Nil finally newLineOpt()
+        def extParamss() = try paramClause(0, prefix = true) :: Nil finally newLineOpt()
         val (leadingTparams, leadingVparamss, flags) =
           if in.token == LBRACKET then
-            (typeParamClause(ParamOwner.Def), extParamss, Method | Extension)
+            (typeParamClause(ParamOwner.Def), extParamss(), Method | Extension)
           else if in.token == LPAREN then
-            (Nil, extParamss, Method | Extension)
+            (Nil, extParamss(), Method | Extension)
           else
             (Nil, Nil, Method)
         val mods1 = addFlag(mods, flags)

docs/docs/reference/contextual/extension-methods.md

@@ -161,8 +161,10 @@ def [T: Numeric](x: T) + (y: T): T =
   summon[Numeric[T]].plus(x, y)
 ```
 
-As usual, type parameters of the extension method follow the defined method name. Nevertheless, such type parameters can already be used in the preceding parameter clause.
-
+If an extension method has type parameters, they come immediately after the `def` and are followed by the extended parameter. When calling a  generic extension method, any explicitly given type arguments follow the method name. So the `second` method can be instantiated as follows:
+```scala
+List(1, 2, 3).second[Int]
+```
 
 ### Syntax