From ddf24b55537b6d08daf554112c6dc82014ee94e0 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jakub=20Cie=C5=9Bluk?=
 <72950514+jkciesluk@users.noreply.github.com>
Date: Fri, 2 Feb 2024 09:28:04 +0100
Subject: [PATCH] chore: Backport changes from Metals (#19592)

backports https://github.com/scalameta/metals/pull/5346
[Cherry-picked f15bbdad89e1a32b66f2c4c7bfae1048f13313e1]
---
 .../pc/completions/MatchCaseCompletions.scala | 32 +++++++++++++++----
 .../completion/CompletionCaseSuite.scala      | 18 ++++++++++-
 2 files changed, 42 insertions(+), 8 deletions(-)

diff --git a/presentation-compiler/src/main/dotty/tools/pc/completions/MatchCaseCompletions.scala b/presentation-compiler/src/main/dotty/tools/pc/completions/MatchCaseCompletions.scala
index 063ed2b44225..45f303316818 100644
--- a/presentation-compiler/src/main/dotty/tools/pc/completions/MatchCaseCompletions.scala
+++ b/presentation-compiler/src/main/dotty/tools/pc/completions/MatchCaseCompletions.scala
@@ -164,13 +164,17 @@ object CaseKeywordCompletion:
             (si, label)
           }
       }
-      val caseItems = res.map((si, label) =>
-        completionGenerator.toCompletionValue(
-          si.sym,
-          label,
-          autoImportsGen.renderImports(si.importSel.toList)
-        )
-      )
+      val caseItems =
+        if res.isEmpty then completionGenerator.caseKeywordOnly
+        else
+          res.map((si, label) =>
+            completionGenerator.toCompletionValue(
+              si.sym,
+              label,
+              autoImportsGen.renderImports(si.importSel.toList),
+            )
+          )
+
       includeExhaustive match
         // In `List(foo).map { cas@@} we want to provide also `case (exhaustive)` completion
         // which works like exhaustive match.
@@ -447,6 +451,20 @@ class CompletionValueGenerator(
     end if
   end labelForCaseMember
 
+  def caseKeywordOnly: List[CompletionValue.Keyword] =
+    if patternOnly.isEmpty then
+      val label = "case"
+      val suffix =
+        if clientSupportsSnippets then " $0 =>"
+        else " "
+      List(
+        CompletionValue.Keyword(
+          label,
+          Some(label + suffix),
+        )
+      )
+    else Nil
+
   def toCompletionValue(
       denot: Denotation,
       label: String,
diff --git a/presentation-compiler/test/dotty/tools/pc/tests/completion/CompletionCaseSuite.scala b/presentation-compiler/test/dotty/tools/pc/tests/completion/CompletionCaseSuite.scala
index 96a7cff9e73c..7a00c0397644 100644
--- a/presentation-compiler/test/dotty/tools/pc/tests/completion/CompletionCaseSuite.scala
+++ b/presentation-compiler/test/dotty/tools/pc/tests/completion/CompletionCaseSuite.scala
@@ -542,7 +542,9 @@ class CompletionCaseSuite extends BaseCompletionSuite:
         |    ca@@
         |  }
         |}""".stripMargin,
-      ""
+      """
+        |case
+        |""".stripMargin
     )
 
   @Test def `private-member-2` =
@@ -722,3 +724,17 @@ class CompletionCaseSuite extends BaseCompletionSuite:
          |""".stripMargin,
       "case (Int, Int) => scala",
     )
+  
+  @Test def `keyword-only` =
+    check(
+      """
+        |sealed trait Alpha
+        |object A {
+        |  List.empty[Alpha].groupBy{
+        |    ca@@
+        |  }
+        |}
+        |""".stripMargin,
+      "case",
+    )
+