diff --git a/compiler/src/dotty/tools/dotc/ast/Desugar.scala b/compiler/src/dotty/tools/dotc/ast/Desugar.scala
index d3b4083c9105..2adb4886ac07 100644
--- a/compiler/src/dotty/tools/dotc/ast/Desugar.scala
+++ b/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 =>
diff --git a/tests/neg/i7529.scala b/tests/neg/i7529.scala
new file mode 100644
index 000000000000..efdaca97c40b
--- /dev/null
+++ b/tests/neg/i7529.scala
@@ -0,0 +1,4 @@
+given fooOps: [A](a: A) with
+
+  @nonsense    // error: not found: nonsense
+  def foo = ???
\ No newline at end of file