Implement auto tupling of function arguments

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

@@ -588,6 +588,26 @@ object desugar {
     Function(params, Match(selector, cases))
   }
 
+  /** Map n-ary function `(p1, ..., pn) => body` where n != 1 to unary function as follows:
+   *
+   *    x$1 => {
+   *      def p1 = x$1._1
+   *      ...
+   *      def pn = x$1._n
+   *      body
+   *    }
+   */
+  def makeTupledFunction(params: List[ValDef], body: Tree)(implicit ctx: Context): Tree = {
+    val param = makeSyntheticParameter()
+    def selector(n: Int) = Select(refOfDef(param), nme.selectorName(n))
+    val vdefs =
+      params.zipWithIndex.map{
+        case (param, idx) =>
+          DefDef(param.name, Nil, Nil, TypeTree(), selector(idx)).withPos(param.pos)
+      }
+    Function(param :: Nil, Block(vdefs, body))
+  }
+
   /** Add annotation with class `cls` to tree:
    *      tree @cls
    */

src/dotty/tools/dotc/transform/DropEmptyCompanions.scala

@@ -40,7 +40,7 @@ class DropEmptyCompanions extends MiniPhaseTransform { thisTransform =>
       case TypeDef(_, impl: Template) if tree.symbol.is(SyntheticModule) &&
         tree.symbol.companionClass.exists &&
         impl.body.forall(_.symbol.isPrimaryConstructor) =>
-        println(i"removing ${tree.symbol}")
+        ctx.log(i"removing ${tree.symbol}")
         true
       case _ =>
         false

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

@@ -611,26 +611,44 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
         if (protoFormals.length == params.length) protoFormals(i)
         else errorType(i"wrong number of parameters, expected: ${protoFormals.length}", tree.pos)
 
-      val inferredParams: List[untpd.ValDef] =
-        for ((param, i) <- params.zipWithIndex) yield
-          if (!param.tpt.isEmpty) param
-          else cpy.ValDef(param)(
-            tpt = untpd.TypeTree(
-              inferredParamType(param, protoFormal(i)).underlyingIfRepeated(isJava = false)))
-
-      // Define result type of closure as the expected type, thereby pushing
-      // down any implicit searches. We do this even if the expected type is not fully
-      // defined, which is a bit of a hack. But it's needed to make the following work
-      // (see typers.scala and printers/PlainPrinter.scala for examples).
-      //
-      //     def double(x: Char): String = s"$x$x"
-      //     "abc" flatMap double
-      //
-      val resultTpt = protoResult match {
-        case WildcardType(_) => untpd.TypeTree()
-        case _ => untpd.TypeTree(protoResult)
+      /** Is `formal` a product type which is elementwise compatible with `params`? */
+      def ptIsCorrectProduct(formal: Type) = {
+        val pclass = defn.ProductNType(params.length).symbol
+        isFullyDefined(formal, ForceDegree.noBottom) &&
+        formal.derivesFrom(pclass) &&
+        formal.baseArgTypes(pclass).corresponds(params) {
+          (argType, param) =>
+            param.tpt.isEmpty || argType <:< typedAheadType(param.tpt).tpe
+        }
       }
-      typed(desugar.makeClosure(inferredParams, fnBody, resultTpt), pt)
+
+      val desugared =
+        if (protoFormals.length == 1 && params.length != 1 && ptIsCorrectProduct(protoFormals.head)) {
+          desugar.makeTupledFunction(params, fnBody)
+        }
+        else {
+          val inferredParams: List[untpd.ValDef] =
+            for ((param, i) <- params.zipWithIndex) yield
+              if (!param.tpt.isEmpty) param
+              else cpy.ValDef(param)(
+                tpt = untpd.TypeTree(
+                  inferredParamType(param, protoFormal(i)).underlyingIfRepeated(isJava = false)))
+
+          // Define result type of closure as the expected type, thereby pushing
+          // down any implicit searches. We do this even if the expected type is not fully
+          // defined, which is a bit of a hack. But it's needed to make the following work
+          // (see typers.scala and printers/PlainPrinter.scala for examples).
+          //
+          //     def double(x: Char): String = s"$x$x"
+          //     "abc" flatMap double
+          //
+          val resultTpt = protoResult match {
+            case WildcardType(_) => untpd.TypeTree()
+            case _ => untpd.TypeTree(protoResult)
+          }
+          desugar.makeClosure(inferredParams, fnBody, resultTpt)
+        }
+      typed(desugared, pt)
     }
   }
 

test/dotc/tests.scala

@@ -111,6 +111,7 @@ class tests extends CompilerTest {
   @Test def neg_abstractOverride() = compileFile(negDir, "abstract-override", xerrors = 2)
   @Test def neg_blockescapes() = compileFile(negDir, "blockescapesNeg", xerrors = 1)
   @Test def neg_bounds() = compileFile(negDir, "bounds", xerrors = 2)
+  @Test def neg_functionArity() = compileFile(negDir, "function-arity", xerrors = 7)
   @Test def neg_typedapply() = compileFile(negDir, "typedapply", xerrors = 3)
   @Test def neg_typedIdents() = compileDir(negDir, "typedIdents", xerrors = 2)
   @Test def neg_assignments() = compileFile(negDir, "assignments", xerrors = 3)

tests/neg/function-arity.scala

@@ -0,0 +1,28 @@
+object Test {
+
+  // From #873:
+
+  trait X extends Function1[Int, String]
+  implicit def f2x(f: Function1[Int, String]): X = ???
+  ({case _ if "".isEmpty => 0} : X) // error: expected String, found Int
+
+  // Tests where parameter list cannot be made into a pattern
+
+  def unary[T](x: T => Unit) = ???
+  unary((x, y) => ())   // error
+
+  unary[(Int, Int)]((x, y) => ())
+
+  unary[(Int, Int)](() => ())   // error
+  unary[(Int, Int)]((x, y, _) => ()) // error
+
+  unary[(Int, Int)]((x: String, y) => ()) // error
+
+  def foo(a: Tuple2[Int, Int] => String): String = ""
+  def foo(a: Any => String) = ()
+  foo((a: Int, b: String) => a + b) // error: none of the overloaded alternatives of method foo match arguments (Int, Int)
+}
+object jasonComment {
+  implicit def i2s(i: Int): String = i.toString
+  ((x: String, y: String) => 42) : (((Int, Int)) => String) // error
+}

tests/pos/i873.scala → tests/pos/function-arity.scala

@@ -7,4 +7,14 @@ object Test {
   ({case _ if "".isEmpty => ""} : X) // allowed, implicit view used to adapt
 
   // ({case _ if "".isEmpty => 0} : X) // expected String, found Int
+
+  def unary[T](a: T, b: T, f: ((T, T)) => T): T = f((a, b))
+  unary(1, 2, (x, y) => x)
+  unary(1, 2, (x: Int, y) => x)
+  unary(1, 2, (x: Int, y: Int) => x)
+
+  val xs = List(1, 2, 3)
+  def f(x: Int, y: Int) = x * y
+  xs.zipWithIndex.map(_ + _)
+  xs.zipWithIndex.map(f)
 }

tests/run/function-arity.scala

@@ -0,0 +1,8 @@
+object Test {
+  class T[A] { def foo(f: (=> A) => Int) = f(???) }
+
+  def main(args: Array[String]): Unit = {
+    new T[(Int, Int)].foo((ii) => 0)
+    new T[(Int, Int)].foo((x, y) => 0) // check that this does not run into ???
+  }
+}