Don't allow two type parameter sections for extension methods

Don't allow extension methods to have type parameters if collective
type parameters were already given. The reason to disallow is that
we don't support multiple consecutive type argument sections yet.
And merging two type parameter sections into one would break a future
extension where we do allow multiple consecutive type argument sections.

Author: odersky

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

@@ -3372,11 +3372,14 @@ object Parsers {
       case _ =>
         syntaxError(em"extension clause must start with a single regular parameter", start)
 
-    def checkExtensionMethod(stat: Tree): Unit = stat match {
+    def checkExtensionMethod(tparams: List[Tree], stat: Tree): Unit = stat match {
       case stat: DefDef =>
         if stat.mods.is(Extension) then
           syntaxError(i"no extension method allowed here since leading parameter was already given", stat.span)
-      case _ =>
+        else if tparams.nonEmpty && stat.tparams.nonEmpty then
+          syntaxError(i"extension method cannot have type parameters since some were already given previously",
+            stat.tparams.head.span)
+      case stat =>
         syntaxError(i"extension clause can only define methods", stat.span)
     }
 
@@ -3406,7 +3409,7 @@ object Parsers {
           possibleTemplateStart()
           val templ = templateBodyOpt(
             makeConstructor(tparams, extParams :: givenParamss), Nil, Nil)
-          templ.body.foreach(checkExtensionMethod)
+          templ.body.foreach(checkExtensionMethod(tparams, _))
           ModuleDef(name, templ)
         else
           var tparams: List[TypeDef] = Nil
@@ -3467,7 +3470,7 @@ object Parsers {
                 vparam.withMods(vparam.mods &~ Param | ParamAccessor | PrivateLocal)))
             val templ = templateBodyOpt(makeConstructor(tparams, vparamss), parents, Nil)
             if hasExtensionParams then
-              templ.body.foreach(checkExtensionMethod)
+              templ.body.foreach(checkExtensionMethod(tparams, _))
               ModuleDef(name, templ)
             else if tparams.isEmpty && vparamss.isEmpty then ModuleDef(name, templ)
             else TypeDef(name.toTypeName, templ)

tests/neg/i6900.scala

@@ -0,0 +1,15 @@
+object Test2 {
+
+  // Works with extension method
+  given extension [A](a: A) with
+    def foo[C]: C => A = _ => a   // error: extension method cannot have type parameters
+
+  1.foo.foo
+
+  // ... but have to pass 2 parameters
+  1.foo.foo[Any => Int, String]
+  1.foo[Int, String].foo
+  1.foo[Int, String].foo[String => Int, String]
+
+}
+