Fix #9562: Fix handling of identifiers in extension methods

Fix 1: Right associative methods

Author: odersky

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

@@ -452,16 +452,14 @@ class Typer extends Namer
      */
     def extensionMethodSelect: untpd.Tree =
       val xmethod = ctx.owner.enclosingExtensionMethod
+      def leadingParam(paramss: List[List[Symbol]]): Symbol = paramss match
+        case (param :: _) :: paramss1 if param.isType => leadingParam(paramss1)
+        case _ :: (snd :: Nil) :: _ if !isLeftAssoc(xmethod.name) => snd
+        case (fst :: Nil) :: _ => fst
+        case _ => NoSymbol
       val qualifier =
-        if xmethod.exists then // TODO: see whether we can use paramss for that
-          val leadParamName = xmethod.info.paramNamess.head.head
-          def isLeadParam(sym: Symbol) =
-            sym.is(Param) && sym.owner.owner == xmethod.owner && sym.name == leadParamName
-          def leadParam(ctx: Context): Symbol =
-            ctx.scope.lookupAll(leadParamName).find(isLeadParam) match
-              case Some(param) => param
-              case None => leadParam(ctx.outersIterator.dropWhile(_.scope eq ctx.scope).next)
-          untpd.ref(leadParam(ctx).termRef)
+        if xmethod.exists then
+          untpd.ref(leadingParam(xmethod.rawParamss).termRef)
         else
           untpd.This(untpd.EmptyTypeIdent)
       untpd.cpy.Select(tree)(qualifier, name)

tests/pos/i9562.scala

@@ -0,0 +1,11 @@
+class Foo:
+  def foo = 23
+
+object Unrelated:
+  extension (f: Foo)
+    def g = f.foo // OK
+
+  extension (f: Foo)
+    def h1: Int = 0
+    def h2: Int = h1 + 1  // OK
+    def ++: (x: Int): Int = h2 + x  // OK