chore: Backport changes from Metals (#19592)

backports scalameta/metals#5346

Author: jkciesluk

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.
@@ -441,6 +445,20 @@ 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,

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",
+    )
+