Fix #7529: Don't drop modifiers when desugaring extension methods

Author: odersky

compiler/src/dotty/tools/dotc/ast/Desugar.scala

@@ -904,7 +904,7 @@ object desugar {
         else cpy.DefDef(mdef)(
           tparams = tparams ++ mdef.tparams,
           vparamss = leadingParams :: givenParamss ::: mdef.vparamss
-        ).withFlags(Extension)
+        ).withMods(mdef.mods | Extension)
       case mdef: Import =>
         mdef
       case mdef =>

tests/neg/i7529.scala

@@ -0,0 +1,4 @@
+given fooOps: [A](a: A) with
+
+  @nonsense    // error: not found: nonsense
+  def foo = ???