Fix of t0438 - lambdas and eta expansion

Two fixes were needed

1) When typing a function value (x1: T1, ..., xN: Tn) => e, don't unconditionally issue an error
   if the expected function type arity is different from N. Instead, issue an error only
   if one of the types T1, ..., Tn is absent. The idea is that only then we need to
   consult the expected type for the parameter type. This allows to fix the problem later
   by an implicit conversion applied to the function value.

2) When eta-expanding, do not automtically take the arity of the expected function value as the
   arity of the generated lambda. Instead, take the method's arity, and copy method parameters
   into the lambda in case the arities are different.

2 files changed, 25 insertions, 14 deletions

diff --git a/src/dotty/tools/dotc/typer/EtaExpansion.scala b/src/dotty/tools/dotc/typer/EtaExpansion.scala
index 46a1d3583..099dd943a 100644
--- a/src/dotty/tools/dotc/typer/EtaExpansion.scala
+++ b/src/dotty/tools/dotc/typer/EtaExpansion.scala
@@ -95,25 +95,35 @@ object EtaExpansion {
 
   /** Eta-expanding a tree means converting a method reference to a function value.
    *  @param    tree       The tree to expand
-   *  @param    paramNames The names of the parameters to use in the expansion
-   *  Let `paramNames` be x1, ..., xn
+   *  @param    mt         The type of the method reference
+   *  @param    xarity     The arity of the expected function type
    *  and assume the lifted application of `tree` (@see liftApp) is
    *
    *         { val xs = es; expr }
    *
-   *  Then the eta-expansion is
+   *  If xarity matches the number of parameters in `mt`, the eta-expansion is
    *
-   *         { val xs = es; (x1, ..., xn) => expr(xx1, ..., xn) }
+   *         { val xs = es; (x1, ..., xn) => expr(x1, ..., xn) }
    *
-   *  This is an untyped tree, with `es` and `expr` as typed splices.
+   * Note that the function value's parameters are untyped, hence the type will
+   * be supplied by the environment (or if missing be supplied by the target
+   * method as a fallback). On the other hand, if `xarity` is different from
+   * the number of parameters in `mt`, then we cannot propagate parameter types
+   * from the expected type, and we fallback to using the method's original
+   * parameter types instead.
+   *
+   * In either case, the result is an untyped tree, with `es` and `expr` as typed splices.
    */
-  def etaExpand(tree: Tree, paramNames: List[TermName])(implicit ctx: Context): untpd.Tree = {
+  def etaExpand(tree: Tree, mt: MethodType, xarity: Int)(implicit ctx: Context): untpd.Tree = {
     import untpd._
     val defs = new mutable.ListBuffer[tpd.Tree]
     val lifted: Tree = TypedSplice(liftApp(defs, tree))
-    val params = paramNames map (name =>
-      ValDef(Modifiers(Synthetic | Param), name, TypeTree(), EmptyTree).withPos(tree.pos))
-    val ids = paramNames map (name =>
+    val paramTypes: List[Tree] =
+      if (mt.paramTypes.length == xarity) mt.paramTypes map (_ => TypeTree())
+      else mt.paramTypes map TypeTree
+    val params = (mt.paramNames, paramTypes).zipped.map((name, tpe) =>
+      ValDef(Modifiers(Synthetic | Param), name, TypeTree(tpe), EmptyTree).withPos(tree.pos))
+    val ids = mt.paramNames map (name =>
       Ident(name).withPos(tree.pos))
     val body = Apply(lifted, ids)
     val fn = untpd.Function(params, body)
diff --git a/src/dotty/tools/dotc/typer/Typer.scala b/src/dotty/tools/dotc/typer/Typer.scala
index 67e5c5902..4fdee86a3 100644
--- a/src/dotty/tools/dotc/typer/Typer.scala
+++ b/src/dotty/tools/dotc/typer/Typer.scala
@@ -496,14 +496,15 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
         errorType(s"missing parameter type for parameter ${param.name}$ofFun, expected = ${pt.show}", param.pos)
       }
 
-      if (protoFormals.length != params.length)
-        ctx.error(s"wrong number of parameters, expected: ${protoFormals.length}", tree.pos)
+      def protoFormal(i: Int): Type =
+        if (protoFormals.length == params.length) protoFormals(i)
+        else errorType(s"wrong number of parameters, expected: ${protoFormals.length}", tree.pos)
 
       val inferredParams: List[untpd.ValDef] =
-        for ((param, formal) <- params zip protoFormals) yield
+        for ((param, i) <- params.zipWithIndex) yield
           if (!param.tpt.isEmpty) param
           else {
-            val paramTpt = untpd.TypeTree(inferredParamType(param, formal))
+            val paramTpt = untpd.TypeTree(inferredParamType(param, protoFormal(i)))
             cpy.ValDef(param, param.mods, param.name, paramTpt, param.rhs)
           }
 
@@ -1135,7 +1136,7 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
           else if (pt eq AnyFunctionProto) wtp.paramTypes.length
           else -1
         if (arity >= 0 && !tree.symbol.isConstructor)
-          typed(etaExpand(tree, wtp.paramNames take arity), pt)
+          typed(etaExpand(tree, wtp, arity), pt)
         else if (wtp.paramTypes.isEmpty)
           adaptInterpolated(tpd.Apply(tree, Nil), pt)
         else