feat: Implement completions for named tuple fields (#21202)

closes scala#20478

---------

Co-authored-by: Jędrzej Rochala <48657087+rochala@users.noreply.github.com>

Author: KacperFKorban

compiler/src/dotty/tools/dotc/interactive/Completion.scala

@@ -32,6 +32,8 @@ import dotty.tools.dotc.core.Names
 import dotty.tools.dotc.core.Types
 import dotty.tools.dotc.core.Symbols
 import dotty.tools.dotc.core.Constants
+import dotty.tools.dotc.core.TypeOps
+import dotty.tools.dotc.core.StdNames
 
 /**
  * One of the results of a completion query.
@@ -200,7 +202,8 @@ object Completion:
 
   private def computeCompletions(
     pos: SourcePosition,
-    mode: Mode, rawPrefix: String,
+    mode: Mode,
+    rawPrefix: String,
     adjustedPath: List[tpd.Tree],
     untpdPath: List[untpd.Tree],
     matches: Option[Name => Boolean]
@@ -442,9 +445,17 @@ object Completion:
     def selectionCompletions(qual: tpd.Tree)(using Context): CompletionMap =
       val adjustedQual = widenQualifier(qual)
 
-      implicitConversionMemberCompletions(adjustedQual) ++
-        extensionCompletions(adjustedQual) ++
-        directMemberCompletions(adjustedQual)
+      val implicitConversionMembers = implicitConversionMemberCompletions(adjustedQual)
+      val extensionMembers = extensionCompletions(adjustedQual)
+      val directMembers = directMemberCompletions(adjustedQual)
+      val namedTupleMembers = namedTupleCompletions(adjustedQual)
+
+      List(
+        implicitConversionMembers,
+        extensionMembers,
+        directMembers,
+        namedTupleMembers
+      ).reduce(_ ++ _)
 
     /** Completions for members of `qual`'s type.
      *  These include inherited definitions but not members added by extensions or implicit conversions
@@ -516,6 +527,30 @@ object Completion:
           .toSeq
           .groupByName
 
+    /** Completions for named tuples */
+    private def namedTupleCompletions(qual: tpd.Tree)(using Context): CompletionMap =
+      def namedTupleCompletionsFromType(tpe: Type): CompletionMap =
+        val freshCtx = ctx.fresh.setExploreTyperState()
+        inContext(freshCtx):
+          tpe.namedTupleElementTypes
+            .map { (name, tpe) =>
+              val symbol = newSymbol(owner = NoSymbol, name, EmptyFlags, tpe)
+              val denot = SymDenotation(symbol, NoSymbol, name, EmptyFlags, tpe)
+              name -> denot
+            }
+            .toSeq
+            .filter((name, denot) => include(denot, name))
+            .groupByName
+
+      val qualTpe = qual.typeOpt
+      if qualTpe.isNamedTupleType then
+        namedTupleCompletionsFromType(qualTpe)
+      else if qualTpe.derivesFrom(defn.SelectableClass) then
+        val pre = if !TypeOps.isLegalPrefix(qualTpe) then Types.SkolemType(qualTpe) else qualTpe
+        val fieldsType = pre.select(StdNames.tpnme.Fields).dealias.simplified
+        namedTupleCompletionsFromType(fieldsType)
+      else Map.empty
+
     /** Completions from extension methods */
     private def extensionCompletions(qual: tpd.Tree)(using Context): CompletionMap =
       def asDefLikeType(tpe: Type): Type = tpe match

language-server/test/dotty/tools/languageserver/CompletionTest.scala

@@ -1723,4 +1723,14 @@ class CompletionTest {
      .completion(m5, Set())
      .completion(m6, Set())
 
+  @Test def namedTupleCompletion: Unit =
+    code"""|import scala.language.experimental.namedTuples
+           |
+           |val person: (name: String, city: String) =
+           |  (name = "Jamie", city = "Lausanne")
+           |
+           |val n = person.na$m1
+           |"""
+      .completion(m1, Set(("name", Field, "String")))
+
 }

presentation-compiler/test/dotty/tools/pc/tests/completion/CompletionSuite.scala

@@ -1983,3 +1983,50 @@ class CompletionSuite extends BaseCompletionSuite:
          |val foo: SomeClass
          |""".stripMargin,
     )
+
+  @Test def `namedTuple completions` =
+    check(
+      """|import scala.language.experimental.namedTuples
+         |import scala.NamedTuple.*
+         |
+         |val person = (name = "Jamie", city = "Lausanne")
+         |
+         |val n = person.na@@""".stripMargin,
+      "name: String",
+      filter = _.contains("name")
+    )
+
+  @Test def `Selectable with namedTuple Fields member` =
+    check(
+      """|import scala.language.experimental.namedTuples
+         |import scala.NamedTuple.*
+         |
+         |class NamedTupleSelectable extends Selectable {
+         |  type Fields <: AnyNamedTuple
+         |  def selectDynamic(name: String): Any = ???
+         |}
+         |
+         |val person2 = new NamedTupleSelectable {
+         |  type Fields = (name: String, city: String)
+         |}
+         |
+         |val n = person2.na@@""".stripMargin,
+      """|name: String
+         |selectDynamic(name: String): Any
+      """.stripMargin,
+      filter = _.contains("name")
+    )
+
+  @Test def `Selectable without namedTuple Fields mamber` =
+    check(
+      """|class NonNamedTupleSelectable extends Selectable {
+         |  def selectDynamic(name: String): Any = ???
+         |}
+         |
+         |val person2 = new NonNamedTupleSelectable {}
+         |
+         |val n = person2.na@@""".stripMargin,
+      """|selectDynamic(name: String): Any
+      """.stripMargin,
+      filter = _.contains("name")
+    )