Set proper position for ValDefs generated from tuples

Previously, the span for ValDefs generated for tuples would encompas the entire expression, which led to difficulties identifying the exact path to the current position. Now, we set the span to be the same as the name underneath.

Not sure if this is a proper solution, since normally ValDefs ecompans the entire span, but in this case it makes 2 different ValDef have the same span. An alternative solution would be to find the one with point nearer to the current position.

This popped up within the Nightly tests we do in Metals.

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

@@ -1143,10 +1143,11 @@ object desugar {
           ) // no `_`
 
       val ids = for ((named, _) <- vars) yield Ident(named.name)
-      val caseDef = CaseDef(pat, EmptyTree, makeTuple(ids))
       val matchExpr =
         if (tupleOptimizable) rhs
-        else Match(makeSelector(rhs, MatchCheck.IrrefutablePatDef), caseDef :: Nil)
+        else
+          val caseDef = CaseDef(pat, EmptyTree, makeTuple(ids)) 
+          Match(makeSelector(rhs, MatchCheck.IrrefutablePatDef), caseDef :: Nil)
       vars match {
         case Nil if !mods.is(Lazy) =>
           matchExpr
@@ -1166,8 +1167,10 @@ object desugar {
           val restDefs =
             for (((named, tpt), n) <- vars.zipWithIndex if named.name != nme.WILDCARD)
             yield
-              if (mods.is(Lazy)) derivedDefDef(original, named, tpt, selector(n), mods &~ Lazy)
-              else derivedValDef(original, named, tpt, selector(n), mods)
+              val valDef = 
+                if (mods.is(Lazy)) derivedDefDef(original, named, tpt, selector(n), mods &~ Lazy)
+                else derivedValDef(original, named, tpt, selector(n), mods)
+              valDef.withSpan(named.span)
           flatTree(firstDef :: restDefs)
       }
   }

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

@@ -234,7 +234,7 @@ class HoverTest {
 
   @Test def tuple: Unit = {
     code"""|object A:
-           |  val (${m1}first${m2}, second) = (1, 2)""".withSource
-      .hover(m1 to m2, hoverContent("Int"))
+           |  val (${m1}first${m2}, second) = (1.0, 2)""".withSource
+      .hover(m1 to m2, hoverContent("Double"))
   }
 }