Add missing case for finding default getters

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

@@ -210,6 +210,7 @@ object Applications {
         case Select(receiver, _) => receiver
         case mr => mr.tpe.normalizedPrefix match {
           case mr: TermRef => ref(mr)
+          case mr: ThisType => singleton(mr)
           case mr =>
             if testOnly then
               // In this case it is safe to skolemize now; we will produce a stable prefix for the actual call.

tests/pos/i12897/A.scala

@@ -0,0 +1,5 @@
+
+package TestFoo
+extension (text : String) def foo(defaultArg : Boolean = true) : String = ""
+
+//def foo(text : String) (defaultArg : Boolean = true) : String = ""

tests/pos/i12897/B.scala

@@ -0,0 +1,4 @@
+
+package TestFoo
+
+val f = "123".foo() //error