Add selection range for a parameter list in a function/method

Given a query as
```scala
def func(a@@: Int, b: Int)(c: Int) =
    a + b + c
```
range selection will now let you choose `a: Int, b: Int`.
The previous options are `a: Int` and the whole definition of `func`.

Author: natsukagami

presentation-compiler/src/main/dotty/tools/pc/SelectionRangeProvider.scala

@@ -6,10 +6,12 @@ import java.util as ju
 import scala.jdk.CollectionConverters._
 import scala.meta.pc.OffsetParams
 
+import dotty.tools.dotc.ast.tpd
 import dotty.tools.dotc.core.Contexts.Context
 import dotty.tools.dotc.interactive.Interactive
 import dotty.tools.dotc.interactive.InteractiveDriver
 import dotty.tools.dotc.util.SourceFile
+import dotty.tools.dotc.util.SourcePosition
 import dotty.tools.pc.utils.MtagsEnrichments.*
 
 import org.eclipse.lsp4j
@@ -46,11 +48,7 @@ class SelectionRangeProvider(
         Interactive.pathTo(driver.openedTrees(uri), pos)(using ctx)
 
       val bareRanges = path
-        .map { tree =>
-          val selectionRange = new SelectionRange()
-          selectionRange.setRange(tree.sourcePos.toLsp)
-          selectionRange
-        }
+        .flatMap(selectionRangesFromTree(pos))
 
       val comments =
         driver.compilationUnits.get(uri).map(_.comments).toList.flatten
@@ -79,6 +77,33 @@ class SelectionRangeProvider(
     }
   end selectionRange
 
+  /** Given a tree, create a seq of [[SelectionRange]]s corresponding to that tree. */
+  private def selectionRangesFromTree(pos: SourcePosition)(tree: tpd.Tree)(using Context) =
+    def toSelectionRange(srcPos: SourcePosition) =
+      val selectionRange = new SelectionRange()
+      selectionRange.setRange(srcPos.toLsp)
+      selectionRange
+
+    val treeSelectionRange = toSelectionRange(tree.sourcePos)
+
+    tree match
+      case tpd.DefDef(name, paramss, tpt, rhs) =>
+        // If source position is within a parameter list, add a selection range covering that whole list.
+        val selectedParams =
+          paramss
+            .iterator
+            .flatMap: // parameter list to a sourcePosition covering the whole list
+              case Seq(param) => Some(param.sourcePos)
+              case params @ Seq(head, tail*) =>
+                val srcPos = head.sourcePos
+                val lastSpan = tail.last.span
+                Some(SourcePosition(srcPos.source, srcPos.span union lastSpan, srcPos.outer))
+              case Seq() => None
+            .find(_.contains(pos))
+            .map(toSelectionRange)
+        selectedParams ++ Seq(treeSelectionRange)
+      case _ => Seq(treeSelectionRange)
+
   private def setParent(
       child: SelectionRange,
       parent: SelectionRange