improvement: Add case keyword completion

mtags/src/main/scala-2/scala/meta/internal/pc/completions/MatchCaseCompletions.scala

@@ -181,7 +181,10 @@ trait MatchCaseCompletions { this: MetalsGlobal =>
           }
         }
         val members = result.result()
-        val edits = members.map(_._2)
+        val edits = {
+          if (members.isEmpty) completionGenerator.caseKeywordOnly
+          else members.map(_._2)
+        }
         // In `List(foo).map { cas@@} we want to provide also `case (exhaustive)` completion
         // which works like exhaustive match, so we need to collect only members from this step
         includeExhaustive match {
@@ -462,6 +465,22 @@ trait MatchCaseCompletions { this: MetalsGlobal =>
       )
     }
 
+    def caseKeywordOnly: List[TextEditMember] =
+      if (patternOnly.isEmpty) {
+        val label = "case"
+        val suffix =
+          if (clientSupportsSnippets) " $0 =>"
+          else " "
+        List(
+          new TextEditMember(
+            label,
+            new l.TextEdit(editRange, label + suffix),
+            NoSymbol.newErrorSymbol(TermName("case")).setInfo(NoType),
+            label = Some(label)
+          )
+        )
+      } else Nil
+
     private def tryInfixPattern(sym: Symbol): Option[String] = {
       sym.primaryConstructor.paramss match {
         case (a :: b :: Nil) :: Nil =>

mtags/src/main/scala-3/scala/meta/internal/pc/completions/MatchCaseCompletions.scala

@@ -165,13 +165,17 @@ 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.
@@ -446,6 +450,20 @@ 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(
       sym: Symbol,
       label: String,

tests/cross/src/test/scala/tests/pc/CompletionCaseSuite.scala

@@ -581,7 +581,7 @@ class CompletionCaseSuite extends BaseCompletionSuite {
   )
 
   check(
-    "private-member".tag(IgnoreScala2),
+    "private-member".tag(IgnoreScala2.and(IgnoreForScala3CompilerPC)),
     """
       |package example
       |import scala.collection.immutable.Vector
@@ -591,7 +591,8 @@ class CompletionCaseSuite extends BaseCompletionSuite {
       |    ca@@  
       |  }
       |}""".stripMargin,
-    ""
+    """|case
+       |""".stripMargin
   )
 
   check(
@@ -779,4 +780,17 @@ class CompletionCaseSuite extends BaseCompletionSuite {
     "case (Int, Int) => scala"
   )
 
+  check(
+    "keyword-only".tag(IgnoreForScala3CompilerPC),
+    """
+      |sealed trait Alpha
+      |object A {
+      |  List.empty[Alpha].groupBy{
+      |    ca@@
+      |  }
+      |}""".stripMargin,
+    """|case
+       |""".stripMargin
+  )
+
 }