Fix scala#3171: Fix handling default getters in secondary constructors

Previous logic would place them in the enclosing class instead of in
the companion object where they belong.

Also, define position for error that checks against multiple overloaded
alternatives with default arguments.

Author: odersky

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

@@ -149,7 +149,7 @@ object desugar {
    *      def f$default$1[T] = 1
    *      def f$default$2[T](x: Int) = x + "m"
    */
-  def defDef(meth: DefDef, isPrimaryConstructor: Boolean = false)(implicit ctx: Context): Tree = {
+  private def defDef(meth: DefDef, isPrimaryConstructor: Boolean = false)(implicit ctx: Context): Tree = {
     val DefDef(name, tparams, vparamss, tpt, rhs) = meth
     val mods = meth.mods
     val epbuf = new ListBuffer[ValDef]
@@ -254,11 +254,17 @@ object desugar {
         .withFlags((mods.flags & AccessFlags).toCommonFlags)
         .withMods(Nil)
 
-    val (constr1, defaultGetters) = defDef(constr0, isPrimaryConstructor = true) match {
-      case meth: DefDef => (meth, Nil)
-      case Thicket((meth: DefDef) :: defaults) => (meth, defaults)
+    var defaultGetters: List[Tree] = Nil
+
+    def decompose(ddef: Tree): DefDef = ddef match {
+      case meth: DefDef => meth
+      case Thicket((meth: DefDef) :: defaults) =>
+        defaultGetters = defaults
+        meth
     }
 
+    val constr1 = decompose(defDef(constr0, isPrimaryConstructor = true))
+
     // The original type and value parameters in the constructor already have the flags
     // needed to be type members (i.e. param, and possibly also private and local unless
     // prefixed by type or val). `tparams` and `vparamss` are the type parameters that
@@ -299,9 +305,11 @@ object desugar {
     // to auxiliary constructors
     val normalizedBody = impl.body map {
       case ddef: DefDef if ddef.name.isConstructorName =>
-        addEvidenceParams(
-          cpy.DefDef(ddef)(tparams = constrTparams),
-          evidenceParams(constr1).map(toDefParam))
+        decompose(
+          defDef(
+            addEvidenceParams(
+              cpy.DefDef(ddef)(tparams = constrTparams),
+              evidenceParams(constr1).map(toDefParam))))
       case stat =>
         stat
     }
@@ -680,7 +688,10 @@ object desugar {
   def defTree(tree: Tree)(implicit ctx: Context): Tree = tree match {
     case tree: ValDef => valDef(tree)
     case tree: TypeDef => if (tree.isClassDef) classDef(tree) else tree
-    case tree: DefDef => defDef(tree)
+    case tree: DefDef =>
+
+      if (tree.name.isConstructorName) tree // was already handled by enclosing classDef
+      else defDef(tree)
     case tree: ModuleDef => moduleDef(tree)
     case tree: PatDef => patDef(tree)
   }

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

@@ -625,7 +625,7 @@ trait Checking {
           else doubleDefError(decl, other)
         }
         if ((decl is HasDefaultParams) && (other is HasDefaultParams)) {
-          ctx.error(em"two or more overloaded variants of $decl have default arguments")
+          ctx.error(em"two or more overloaded variants of $decl have default arguments", decl.pos)
           decl resetFlag HasDefaultParams
         }
       }

tests/neg/i3171.scala

@@ -0,0 +1,10 @@
+object Test {
+  class C(x: Int = 1, y: Int) {
+    def this(x: Int = 1)(y: String) = // error: two or more overloaded methods have default getters
+      this(x, y.toInt)
+  }
+
+  def test: Unit = {
+    new C()("1") // error: missing argument
+  }
+}

tests/pos/i3171.scala

@@ -0,0 +1,10 @@
+object Test {
+  class C(x: Int, y: Int) {
+    def this(x: Int = 1)(y: String) =
+      this(x, y.toInt)
+  }
+
+  def test: Unit = {
+    new C()("1")
+  }
+}