fix: make vals created in desugaring of n-ary lambdas non-synthetic (#23896)

resolves: #16110
resolves: scalameta/metals#7594
[Cherry-picked 5f470ea]

Author: kasiaMarek

compiler/src/dotty/tools/dotc/ast/Desugar.scala

@@ -1968,7 +1968,6 @@ object desugar {
           ValDef(param.name, param.tpt, selector(idx))
             .withSpan(param.span)
             .withAttachment(UntupledParam, ())
-            .withFlags(Synthetic)
       }
     Function(param :: Nil, Block(vdefs, body))
   }

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

@@ -120,7 +120,7 @@ object PcConvertToNamedLambdaParameters:
   }
 
   def isWildcardParam(param: tpd.ValDef)(using Context): Boolean =
-    param.name.toString.startsWith("_$") && param.symbol.is(Flags.Synthetic)
+    param.name.toString.startsWith("_$")
 
   def findParamReferencePosition(param: tpd.ValDef, lambda: tpd.Tree)(using Context): Option[SourcePosition] =
     var pos: Option[SourcePosition] = None

presentation-compiler/test/dotty/tools/pc/tests/hover/HoverTermSuite.scala

@@ -6,6 +6,18 @@ import org.junit.Test
 
 class HoverTermSuite extends BaseHoverSuite:
 
+  @Test def `n-ary lamba` =
+    check(
+      """|object testRepor {
+         |  val listOfTuples = List(1 -> 1, 2 -> 2, 3 -> 3)
+         |
+         |  listOfTuples.map((k@@ey, value) => key + value)
+         |}
+         |""".stripMargin,
+      """|val key: Int
+         |""".stripMargin.hover
+    )
+
   @Test def `map` =
     check(
       """object a {