From d4b431651f865988d6de00e7a639784ed3a10144 Mon Sep 17 00:00:00 2001
From: Vadim Chelyshov <qtankle@gmail.com>
Date: Mon, 25 Aug 2025 21:17:44 +0300
Subject: [PATCH] pc: completions - do not add `[]` for `... derives TC@@`

Currently it incorectly adds completion members with square brackets.
Exmaple:
```scala
class X derives CanEqua@@ // returns `CanEqual[@@]` and `CanEqual`
                          // should return only `CanEqual`
```
---
 .../main/dotty/tools/pc/completions/Completions.scala  | 10 +++++++++-
 .../tools/pc/tests/completion/CompletionSuite.scala    |  8 ++++++++
 2 files changed, 17 insertions(+), 1 deletion(-)

diff --git a/presentation-compiler/src/main/dotty/tools/pc/completions/Completions.scala b/presentation-compiler/src/main/dotty/tools/pc/completions/Completions.scala
index 78130e7d9fd5..ea3565574ce3 100644
--- a/presentation-compiler/src/main/dotty/tools/pc/completions/Completions.scala
+++ b/presentation-compiler/src/main/dotty/tools/pc/completions/Completions.scala
@@ -74,7 +74,15 @@ class Completions(
       case tpe :: (appl: AppliedTypeTree) :: _ if appl.tpt == tpe => false
       case sel  :: (funSel @ Select(fun, name)) :: (appl: GenericApply) :: _
         if appl.fun == funSel && sel == fun => false
-      case _ => true)
+      case _ => true) &&
+      (adjustedPath match
+        /* In case of `class X derives TC@@` we shouldn't add `[]` 
+         */
+        case Ident(_) :: (templ: untpd.DerivingTemplate) :: _ =>
+          val pos = completionPos.toSourcePosition
+          !templ.derived.exists(_.sourcePos.contains(pos))
+        case _ => true
+      )
 
   private lazy val isNew: Boolean = Completion.isInNewContext(adjustedPath)
 
diff --git a/presentation-compiler/test/dotty/tools/pc/tests/completion/CompletionSuite.scala b/presentation-compiler/test/dotty/tools/pc/tests/completion/CompletionSuite.scala
index 277a579ba4ce..4dad18a78181 100644
--- a/presentation-compiler/test/dotty/tools/pc/tests/completion/CompletionSuite.scala
+++ b/presentation-compiler/test/dotty/tools/pc/tests/completion/CompletionSuite.scala
@@ -2285,3 +2285,11 @@ class CompletionSuite extends BaseCompletionSuite:
         |""".stripMargin,
       "asTerm: Term"
     )
+  
+  @Test def `derives-no-square-brackets` =
+    check(
+      """
+        |case class Miau(y: Int) derives Ordering, CanEqu@@
+        |""".stripMargin,
+      "CanEqual scala"
+    )