Fix scala#8717: Add missing extension span

Author: nicolasstucki

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

@@ -3588,7 +3588,7 @@ object Parsers {
         if !in.isNestedStart then syntaxError("Extension without extension methods")
         val templ = templateBodyOpt(makeConstructor(tparams, vparamss), Nil, Nil)
         templ.body.foreach(checkExtensionMethod(tparams, vparamss, _))
-        val edef = ModuleDef(name, templ)
+        val edef = atSpan(start, in.offset)(ModuleDef(name, templ))
         finalizeDef(edef, addFlag(mods, Given | extensionFlag), start)
       }
 

compiler/test-resources/repl/i8717

@@ -0,0 +1,4 @@
+scala>extension { }
+1 |extension { }
+  |^
+  |anonymous instance must implement a type or have at least one extension method