bugfix: suggest correct arg name completions for lambda expressions (#18379)

backport from metals: scalameta/metals#5533
CC: @tgodzik

Author: tgodzik

presentation-compiler/src/main/dotty/tools/pc/completions/CompletionValue.scala

@@ -247,10 +247,10 @@ object CompletionValue:
       description
     override def insertMode: Option[InsertTextMode] = Some(InsertTextMode.AsIs)
 
-  def namedArg(label: String, sym: Symbol)(using
+  def namedArg(label: String, sym: ParamSymbol)(using
       Context
   ): CompletionValue =
-    NamedArg(label, sym.info.widenTermRefExpr, sym)
+    NamedArg(label, sym.info.widenTermRefExpr, sym.symbol)
 
   def keyword(label: String, insertText: String): CompletionValue =
     Keyword(label, Some(insertText))

presentation-compiler/src/main/dotty/tools/pc/completions/NamedArgCompletions.scala

@@ -16,14 +16,17 @@ import dotty.tools.dotc.core.Symbols
 import dotty.tools.dotc.core.Symbols.Symbol
 import dotty.tools.dotc.core.Types.AndType
 import dotty.tools.dotc.core.Types.AppliedType
+import dotty.tools.dotc.core.Types.MethodType
 import dotty.tools.dotc.core.Types.OrType
+import dotty.tools.dotc.core.Types.RefinedType
 import dotty.tools.dotc.core.Types.TermRef
 import dotty.tools.dotc.core.Types.Type
 import dotty.tools.dotc.core.Types.TypeBounds
 import dotty.tools.dotc.core.Types.WildcardType
 import dotty.tools.dotc.util.SourcePosition
 import dotty.tools.pc.IndexedContext
 import dotty.tools.pc.utils.MtagsEnrichments.*
+import scala.annotation.tailrec
 
 object NamedArgCompletions:
 
@@ -195,9 +198,40 @@ object NamedArgCompletions:
       // def curry(x: Int)(apple: String, banana: String) = ???
       // curry(1)(apple = "test", b@@)
       // ```
-      val (baseParams, baseArgs) =
+      val (baseParams0, baseArgs) =
         vparamss.zip(argss).lastOption.getOrElse((Nil, Nil))
 
+      val baseParams: List[ParamSymbol] =
+        def defaultBaseParams = baseParams0.map(JustSymbol(_))
+        @tailrec
+        def getRefinedParams(refinedType: Type, level: Int): List[ParamSymbol] =
+          if level > 0 then
+            val resultTypeOpt =
+              refinedType match
+                case RefinedType(AppliedType(_, args), _, _) => args.lastOption
+                case AppliedType(_, args) => args.lastOption
+                case _ => None
+            resultTypeOpt match
+              case Some(resultType) => getRefinedParams(resultType, level - 1)
+              case _ => defaultBaseParams
+          else
+            refinedType match
+              case RefinedType(AppliedType(_, args), _, MethodType(ri)) =>
+                baseParams0.zip(ri).zip(args).map { case ((sym, name), arg) =>
+                  RefinedSymbol(sym, name, arg)
+                }
+              case _ => defaultBaseParams
+        // finds param refinements for lambda expressions
+        // val hello: (x: Int, y: Int) => Unit = (x, _) => println(x)
+        @tailrec
+        def refineParams(method: Tree, level: Int): List[ParamSymbol] =
+          method match
+            case Select(Apply(f, _), _) => refineParams(f, level + 1)
+            case Select(h, v) => getRefinedParams(h.symbol.info, level)
+            case _ => defaultBaseParams
+        refineParams(method, 0)
+      end baseParams
+
       val args = ident
         .map(i => baseArgs.filterNot(_ == i))
         .getOrElse(baseArgs)
@@ -221,7 +255,7 @@ object NamedArgCompletions:
 
       baseParams.filterNot(param =>
         isNamed(param.name) ||
-          param.denot.is(
+          param.symbol.denot.is(
             Flags.Synthetic
           ) // filter out synthesized param, like evidence
       )
@@ -232,7 +266,7 @@ object NamedArgCompletions:
         .map(_.name.toString)
         .getOrElse("")
         .replace(Cursor.value, "")
-    val params: List[Symbol] =
+    val params: List[ParamSymbol] =
       allParams
         .filter(param => param.name.startsWith(prefix))
         .distinctBy(sym => (sym.name, sym.info))
@@ -249,7 +283,7 @@ object NamedArgCompletions:
         .filter(name => name != "Nil" && name != "None")
         .sorted
 
-    def findDefaultValue(param: Symbol): String =
+    def findDefaultValue(param: ParamSymbol): String =
       val matchingType = matchingTypesInScope(param.info)
       if matchingType.size == 1 then s":${matchingType.head}"
       else if matchingType.size > 1 then s"|???,${matchingType.mkString(",")}|"
@@ -260,12 +294,12 @@ object NamedArgCompletions:
       def shouldShow =
         allParams.exists(param => param.name.startsWith(prefix))
       def isExplicitlyCalled = suffix.startsWith(prefix)
-      def hasParamsToFill = allParams.count(!_.is(Flags.HasDefault)) > 1
+      def hasParamsToFill = allParams.count(!_.symbol.is(Flags.HasDefault)) > 1
       if clientSupportsSnippets && matchingMethods.length == 1 && (shouldShow || isExplicitlyCalled) && hasParamsToFill
       then
         val editText = allParams.zipWithIndex
           .collect {
-            case (param, index) if !param.is(Flags.HasDefault) =>
+            case (param, index) if !param.symbol.is(Flags.HasDefault) =>
               s"${param.nameBackticked.replace("$", "$$")} = $${${index + 1}${findDefaultValue(param)}}"
           }
           .mkString(", ")
@@ -355,3 +389,16 @@ class FuzzyArgMatcher(tparams: List[Symbols.Symbol])(using Context):
       case _ => t
 
 end FuzzyArgMatcher
+
+sealed trait ParamSymbol:
+  def name: Name
+  def info: Type
+  def symbol: Symbol
+  def nameBackticked(using Context) = name.decoded.backticked
+
+case class JustSymbol(symbol: Symbol)(using Context) extends ParamSymbol:
+  def name: Name = symbol.name
+  def info: Type = symbol.info
+
+case class RefinedSymbol(symbol: Symbol, name: Name, info: Type)
+    extends ParamSymbol

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

@@ -881,3 +881,55 @@ class CompletionArgSuite extends BaseCompletionSuite:
          |""".stripMargin,
       topLines = Some(1),
     )
+
+
+  @Test def `lambda` =
+    check(
+      """|val hello: (x: Int) => Unit = x => println(x)
+         |val k = hello(@@)
+         |""".stripMargin,
+      """|x = : Int
+         |""".stripMargin,
+      topLines = Some(1),
+    )
+
+  @Test def `lambda2` =
+    check(
+      """|object O:
+         |  val hello: (x: Int, y: Int) => Unit = (x, _) => println(x)
+         |val k = O.hello(x = 1, @@)
+         |""".stripMargin,
+      """|y = : Int
+         |""".stripMargin,
+      topLines = Some(1),
+    )
+
+  @Test def `lambda3` =
+    check(
+      """|val hello: (x: Int) => (j: Int) => Unit = x => j => println(x)
+         |val k = hello(@@)
+         |""".stripMargin,
+      """|x = : Int
+         |""".stripMargin,
+      topLines = Some(1),
+    )
+
+  @Test def `lambda4` =
+    check(
+      """|val hello: (x: Int) => (j: Int) => (str: String) => Unit = x => j => str => println(str)
+         |val k = hello(x = 1)(2)(@@)
+         |""".stripMargin,
+      """|str = : String
+         |""".stripMargin,
+      topLines = Some(1),
+    )
+
+  @Test def `lambda5` =
+    check(
+      """|val hello: (x: Int) => Int => (str: String) => Unit = x => j => str => println(str)
+         |val k = hello(x = 1)(2)(@@)
+         |""".stripMargin,
+      """|str = : String
+         |""".stripMargin,
+      topLines = Some(1),
+    )