Merge pull request #2701 from dotty-staging/fix-#2570

Changes to eta expansion

Author: odersky

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

@@ -474,6 +474,14 @@ trait TypedTreeInfo extends TreeInfo[Type] { self: Trees.Instance[Type] =>
     case _ => false
   }
 
+  /** Is tree a compiler-generated `.apply` node that refers to the
+   *  apply of a function class?
+   */
+  def isSyntheticApply(tree: Tree): Boolean = tree match {
+    case Select(qual, nme.apply) => tree.pos.end == qual.pos.end
+    case _ => false
+  }
+
   /** Strips layers of `.asInstanceOf[T]` / `_.$asInstanceOf[T]()` from an expression */
   def stripCast(tree: Tree)(implicit ctx: Context): Tree = {
     def isCast(sel: Tree) = sel.symbol == defn.Any_asInstanceOf

compiler/src/dotty/tools/dotc/core/Definitions.scala

@@ -809,8 +809,8 @@ class Definitions {
    */
   def erasedFunctionClass(cls: Symbol): Symbol = {
     val arity = scalaClassName(cls).functionArity
-    if (arity > 22) defn.FunctionXXLClass
-    else if (arity >= 0) defn.FunctionClass(arity)
+    if (arity > 22) FunctionXXLClass
+    else if (arity >= 0) FunctionClass(arity)
     else NoSymbol
   }
 

compiler/src/dotty/tools/dotc/reporting/diagnostic/messages.scala

@@ -702,8 +702,7 @@ object messages {
     private val msgPrefix = if (actualCount > expectedCount) "Too many" else "Not enough"
 
     //TODO add def simpleParamName to ParamInfo
-    private val expectedArgString = fntpe
-      .widen.typeParams
+    private val expectedArgString = expectedArgs
       .map(_.paramName.unexpandedName.show)
       .mkString("[", ", ", "]")
 

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

@@ -2006,9 +2006,48 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
               // prioritize method parameter types as parameter types of the eta-expanded closure
               0
             else defn.functionArity(ptNorm)
-          else if (pt eq AnyFunctionProto) wtp.paramInfos.length
-          else -1
-        if (arity >= 0 && !tree.symbol.isConstructor)
+          else {
+            val nparams = wtp.paramInfos.length
+            if (nparams > 0 || pt.eq(AnyFunctionProto)) nparams
+            else -1 // no eta expansion in this case
+          }
+
+        /** A synthetic apply should be eta-expanded if it is the apply of an implicit function
+         *  class, and the expected type is a function type. This rule is needed so we can pass
+         *  an implicit function to a regular function type. So the following is OK
+         *
+         *     val f: implicit A => B  =  ???
+         *     val g: A => B = f
+         *
+         *  and the last line expands to
+         *
+         *     val g: A => B  =  (x$0: A) => f.apply(x$0)
+         *
+         *  One could be tempted not to eta expand the rhs, but that would violate the invariant
+         *  that expressions of implicit function types are always implicit closures, which is
+         *  exploited by ShortcutImplicits.
+         *
+         *  On the other hand, the following would give an error if there is no implicit
+         *  instance of A available.
+         *
+         *     val x: AnyRef = f
+         *
+         *  That's intentional, we want to fail here, otherwise some unsuccesful implicit searches
+         *  would go undetected.
+         *
+         *  Examples for these cases are found in run/implicitFuns.scala and neg/i2006.scala.
+         */
+        def isExpandableApply =
+          defn.isImplicitFunctionClass(tree.symbol.maybeOwner) && defn.isFunctionType(ptNorm)
+
+        // Reasons NOT to eta expand:
+        //  - we reference a constructor
+        //  - we are in a patterm
+        //  - the current tree is a synthetic apply which is not expandable (eta-expasion would simply undo that)
+        if (arity >= 0 &&
+            !tree.symbol.isConstructor &&
+            !ctx.mode.is(Mode.Pattern) &&
+            !(isSyntheticApply(tree) && !isExpandableApply))
           typed(etaExpand(tree, wtp, arity), pt)
         else if (wtp.paramInfos.isEmpty)
           adaptInterpolated(tpd.Apply(tree, Nil), pt, EmptyTree)

tests/neg/i1503.scala

@@ -9,6 +9,6 @@ object Test {
 
   def main(args: Array[String]) = {
     (if (cond) foo1 else bar1)() // error: Unit does not take parameters
-    (if (cond) foo2 else bar2)(22) // error: missing arguments // error: missing arguments
+    (if (cond) foo2 else bar2)(22) // OK after changes to eta expansion
   }
 }

tests/neg/i1716.scala

@@ -1,9 +1,10 @@
 object Fail {
   def f(m: Option[Int]): Unit = {
      m match {
-      case x @ Some[_] =>       // error
+      case x @ Some[_] =>       // error: unbound wildcard type
       case _           =>
     }
   }
-  Some[_]  // error
+  Some[_]  // error: unbound wildcard type
+
 }

tests/neg/typedapply.scala

@@ -6,8 +6,18 @@ object typedapply {
 
   foo[Int, String, String](1, "abc") // error: wrong number of type parameters
 
+  def fuz(x: Int, y: String) = (x, y)
+
+  val x1 = fuz.curried // OK
+  val x2: Int => String => (Int, String) = x1
+
   def bar(x: Int) = x
 
-  bar[Int](1) // error: does not take parameters
+  bar[Int](1) // error: does not take type parameters
+
+  bar.baz // error: baz is not a member of Int => Int
+
 
 }
+
+

tests/pos/i2570.scala

@@ -21,9 +21,18 @@ object Test {
   def sum2(x: Int, ys: Int*) = (x /: ys)(_ + _)
   val h1: ((Int, Seq[Int]) => Int) = sum2
 
-// Not yet:
-//  val h1 = repeat
-//  val h2: (String, Int, Int) = h1
-//  val h3 = sum
-//  val h4: (Int, => Int) = h3
+  val h3 = repeat
+  val h4: ((String, Int, Int) => String) = h3
+  val h5 = sum
+  val h6: ((Int, => Int) => Int) = h5
+
+  class A
+  class B
+  class C
+  implicit object b extends B
+
+  def foo(x: A)(implicit bla: B): C = ???
+
+  val h7 = foo
+  val h8: A => C = h7
 }