From 56e2a31fca6bbd543c74a140e4d3232b0d57b7df Mon Sep 17 00:00:00 2001
From: Dolphin von Chips <td2505052@gmail.com>
Date: Mon, 12 May 2025 18:58:32 +0300
Subject: [PATCH] Disallow empty parameter clauses in `extension` definition

---
 compiler/src/dotty/tools/dotc/parsing/Parsers.scala | 10 ++++++++--
 tests/neg/extensions-can-only-have-using.scala      |  3 +++
 2 files changed, 11 insertions(+), 2 deletions(-)
 create mode 100644 tests/neg/extensions-can-only-have-using.scala

diff --git a/compiler/src/dotty/tools/dotc/parsing/Parsers.scala b/compiler/src/dotty/tools/dotc/parsing/Parsers.scala
index 2a91dc5090bb..9c081c8d21af 100644
--- a/compiler/src/dotty/tools/dotc/parsing/Parsers.scala
+++ b/compiler/src/dotty/tools/dotc/parsing/Parsers.scala
@@ -3673,7 +3673,10 @@ object Parsers {
       // begin termParamClause
       inParensWithCommas {
         if in.token == RPAREN && paramOwner != ParamOwner.ExtensionPrefix && !impliedMods.is(Given)
-        then Nil
+        then
+          if paramOwner.takesOnlyUsingClauses then
+            syntaxError(em"`using` expected")
+          Nil
         else
           val clause =
             if paramOwner == ParamOwner.ExtensionPrefix
@@ -4468,7 +4471,10 @@ object Parsers {
         leadParamss += extParams
         isUsingClause(extParams)
       do ()
-      leadParamss ++= termParamClauses(ParamOwner.ExtensionFollow, numLeadParams)
+      // Empty parameter clauses are filtered out. They are already reported as syntax errors and are not
+      // allowed here.
+      val extFollowParams = termParamClauses(ParamOwner.ExtensionFollow, numLeadParams).filterNot(_.isEmpty)
+      leadParamss ++= extFollowParams
       if in.isColon then
         syntaxError(em"no `:` expected here")
         in.nextToken()
diff --git a/tests/neg/extensions-can-only-have-using.scala b/tests/neg/extensions-can-only-have-using.scala
new file mode 100644
index 000000000000..fcf451fc7258
--- /dev/null
+++ b/tests/neg/extensions-can-only-have-using.scala
@@ -0,0 +1,3 @@
+extension(x: Any)() // error
+  def f = 42
+val x = Nil.f