Fix position of implicit conversions, eta-expansion and module vals

Synthetic trees should get zero-extent positions so that they don't get
returned by Interactive#pathTo. This makes it easier to show information
related to non-synthetic trees in IDEs.

Author: smarter

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

@@ -548,7 +548,7 @@ object desugar {
       val clsRef = Ident(clsName)
       val modul = ValDef(moduleName, clsRef, New(clsRef, Nil))
         .withMods(mods | ModuleCreationFlags | mods.flags & AccessFlags)
-        .withPos(mdef.pos)
+        .withPos(mdef.pos.startPos)
       val ValDef(selfName, selfTpt, _) = impl.self
       val selfMods = impl.self.mods
       if (!selfTpt.isEmpty) ctx.error(ObjectMayNotHaveSelfType(mdef), impl.self.pos)

compiler/src/dotty/tools/dotc/typer/EtaExpansion.scala

@@ -139,8 +139,8 @@ object EtaExpansion {
       if (mt.paramInfos.length == xarity) mt.paramInfos map (_ => TypeTree())
       else mt.paramInfos map TypeTree
     val params = (mt.paramNames, paramTypes).zipped.map((name, tpe) =>
-      ValDef(name, tpe, EmptyTree).withFlags(Synthetic | Param).withPos(tree.pos))
-    var ids: List[Tree] = mt.paramNames map (name => Ident(name).withPos(tree.pos))
+      ValDef(name, tpe, EmptyTree).withFlags(Synthetic | Param).withPos(tree.pos.startPos))
+    var ids: List[Tree] = mt.paramNames map (name => Ident(name).withPos(tree.pos.startPos))
     if (mt.paramInfos.nonEmpty && mt.paramInfos.last.isRepeatedParam)
       ids = ids.init :+ repeated(ids.last)
     var body: Tree = Apply(lifted, ids)

compiler/src/dotty/tools/dotc/typer/Implicits.scala

@@ -743,7 +743,7 @@ trait Implicits { self: Typer =>
       def typedImplicit(cand: Candidate)(implicit ctx: Context): SearchResult = track("typedImplicit") { ctx.traceIndented(i"typed implicit ${cand.ref}, pt = $pt, implicitsEnabled == ${ctx.mode is ImplicitsEnabled}", implicits, show = true) {
         assert(constr eq ctx.typerState.constraint)
         val ref = cand.ref
-        var generated: Tree = tpd.ref(ref).withPos(pos)
+        var generated: Tree = tpd.ref(ref).withPos(pos.startPos)
         if (!argument.isEmpty)
           generated = typedUnadapted(
             untpd.Apply(untpd.TypedSplice(generated), untpd.TypedSplice(argument) :: Nil),