bugfix: Show properly completions for higher order functions

mtags/src/main/scala-2/scala/meta/internal/pc/CompletionProvider.scala

@@ -45,18 +45,29 @@ class CompletionProvider(
 
   def completions(): CompletionList = {
     val filename = params.uri().toString()
-    val unit = addCompilationUnit(
-      code = params.text,
-      filename = filename,
-      cursor = Some(params.offset),
-      cursorName = cursorName
-    )
+    def baseCompletions(
+        // handling issues in `f(_.) where f has more than one parameter`
+        retryWithComa: Boolean = true,
+        addComma: Boolean = false
+    ): (Position, (InterestingMembers, CompletionPosition, l.Range, String)) = {
+      val unit = addCompilationUnit(
+        code = params.text,
+        filename = filename,
+        cursor = Some(params.offset),
+        cursorName = cursorName,
+        withComma = addComma
+      )
 
-    val pos = unit.position(params.offset)
-    val isSnippet = isSnippetEnabled(pos, params.text())
+      val pos = unit.position(params.offset)
 
-    val (i, completion, editRange, query) = safeCompletionsAt(pos, params.uri())
+      val result @ (i, _, _, _) =
+        safeCompletionsAt(pos, params.uri())
+      if (i.results.nonEmpty || !retryWithComa) (pos, result)
+      else baseCompletions(retryWithComa = false, addComma = true)
+    }
 
+    val (pos, (i, completion, editRange, query)) = baseCompletions()
+    val isSnippet = isSnippetEnabled(pos, params.text())
     val start = inferIdentStart(pos, params.text())
     val end = inferIdentEnd(pos, params.text())
     val oldText = params.text().substring(start, end)

mtags/src/main/scala-2/scala/meta/internal/pc/MetalsGlobal.scala

@@ -625,11 +625,23 @@ class MetalsGlobal(
       code: String,
       filename: String,
       cursor: Option[Int],
-      cursorName: String = CURSOR
+      cursorName: String = CURSOR,
+      withComma: Boolean = false
   ): RichCompilationUnit = {
+    // helps to deal with f(_.) where f has a second parameter
+    def nextIsParen(offset: Int): Boolean = {
+      val next = code(offset)
+      if (next == ')') true
+      else if (next == ' ' && offset + 1 < code.size) nextIsParen(offset + 1)
+      else false
+    }
     val codeWithCursor = cursor match {
       case Some(offset) =>
-        code.take(offset) + cursorName + code.drop(offset)
+        val inside =
+          if (withComma && offset < code.size && nextIsParen(offset))
+            cursorName + ","
+          else cursorName
+        code.take(offset) + inside + code.drop(offset)
       case _ => code
     }
     val unit = newCompilationUnit(codeWithCursor, filename)

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

@@ -273,6 +273,25 @@ class CompletionIssueSuite extends BaseCompletionSuite {
     "match"
   )
 
+  check(
+    "higher-order",
+    """|
+       |class Foo {
+       |  def Bar: Double = 2.0
+       |  def Baz: Double = 1.0
+       |}
+       |
+       |object Bar {
+       |  def higherOrder2(f: Foo => Double, s: String): Nothing = ???
+       |  higherOrder2(_.@@)
+       |}
+       |""".stripMargin,
+    """|Bar: Double
+       |Baz: Double
+       |""".stripMargin,
+    topLines = Some(2)
+  )
+
   // The tests shows `x$1` but it's because the dependency is not indexed
   checkEdit(
     "default-java-override",