Fix #7757: Do auto-parameter-untupling also for overloaded methods

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

@@ -1316,33 +1316,39 @@ object desugar {
     Function(params, Match(makeSelector(selector, checkMode), cases))
   }
 
-  /** Map n-ary function `(p1, ..., pn) => body` where n != 1 to unary function as follows:
+  /** Map n-ary function `(x1: T1, ..., xn: Tn) => body` where n != 1 to unary function as follows:
    *
-   *    x$1 => {
-   *      def p1 = x$1._1
+   *    (x$1: (T1, ..., Tn)) => {
+   *      def x1: T1 = x$1._1
    *      ...
-   *      def pn = x$1._n
+   *      def xn: Tn = x$1._n
    *      body
    *    }
    *
    *  or if `isGenericTuple`
    *
-   *    x$1 => {
-   *      def p1 = x$1.apply(0)
+   *    (x$1: (T1, ... Tn) => {
+   *      def x1: T1 = x$1.apply(0)
    *      ...
-   *      def pn = x$1.apply(n-1)
+   *      def xn: Tn = x$1.apply(n-1)
    *      body
    *    }
+   *
+   *  If some of the Ti's are absent, omit the : (T1, ..., Tn) type ascription
+   *  in the selector.
    */
   def makeTupledFunction(params: List[ValDef], body: Tree, isGenericTuple: Boolean)(implicit ctx: Context): Tree = {
-    val param = makeSyntheticParameter()
+    val param = makeSyntheticParameter(
+      tpt =
+        if params.exists(_.tpt.isEmpty) then TypeTree()
+        else Tuple(params.map(_.tpt)))
     def selector(n: Int) =
       if (isGenericTuple) Apply(Select(refOfDef(param), nme.apply), Literal(Constant(n)))
       else Select(refOfDef(param), nme.selectorName(n))
     val vdefs =
       params.zipWithIndex.map {
         case (param, idx) =>
-          DefDef(param.name, Nil, Nil, TypeTree(), selector(idx)).withSpan(param.span)
+          DefDef(param.name, Nil, Nil, param.tpt, selector(idx)).withSpan(param.span)
       }
     Function(param :: Nil, Block(vdefs, body))
   }

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

@@ -114,6 +114,22 @@ object Decorators {
         else x1 :: xs1
       }
 
+    /** Like `xs.lazyZip(xs.indices).map(f)`, but returns list `xs` itself
+     *  - instead of a copy - if function `f` maps all elements of
+     *  `xs` to themselves.
+     */
+    def mapWithIndexConserve[U <: T](f: (T, Int) => U): List[U] =
+      def recur(xs: List[T], idx: Int): List[U] =
+        if xs.isEmpty then Nil
+        else
+          val x1 = f(xs.head, idx)
+          val xs1 = recur(xs.tail, idx + 1)
+          if (x1.asInstanceOf[AnyRef] eq xs.head.asInstanceOf[AnyRef])
+             && (xs1 eq xs.tail)
+          then xs.asInstanceOf[List[U]]
+          else x1 :: xs1
+      recur(xs, 0)
+
     final def hasSameLengthAs[U](ys: List[U]): Boolean = {
       @tailrec def loop(xs: List[T], ys: List[U]): Boolean =
         if (xs.isEmpty) ys.isEmpty

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

@@ -3,7 +3,7 @@ package dotc
 package typer
 
 import core._
-import ast.{Trees, tpd, untpd}
+import ast.{Trees, tpd, untpd, desugar}
 import util.Spans._
 import util.Stats.record
 import util.{SourcePosition, NoSourcePosition, SourceFile}
@@ -864,7 +864,7 @@ trait Applications extends Compatibility {
           case funRef: TermRef =>
             val app =
               if (proto.allArgTypesAreCurrent())
-                new ApplyToTyped(tree, fun1, funRef, proto.unforcedTypedArgs, pt)
+                new ApplyToTyped(tree, fun1, funRef, proto.typedArgs(), pt)
               else
                 new ApplyToUntyped(tree, fun1, funRef, proto, pt)(
                   given fun1.nullableInArgContext(given argCtx(tree)))
@@ -891,7 +891,7 @@ trait Applications extends Compatibility {
           }
 
       fun1.tpe match {
-        case err: ErrorType => cpy.Apply(tree)(fun1, proto.unforcedTypedArgs).withType(err)
+        case err: ErrorType => cpy.Apply(tree)(fun1, proto.typedArgs()).withType(err)
         case TryDynamicCallType => typedDynamicApply(tree, pt)
         case _ =>
           if (originalProto.isDropped) fun1
@@ -1635,14 +1635,46 @@ trait Applications extends Compatibility {
     def narrowByTypes(alts: List[TermRef], argTypes: List[Type], resultType: Type): List[TermRef] =
       alts filter (isApplicableMethodRef(_, argTypes, resultType))
 
+    /** Normalization steps before checking arguments:
+     *
+     *                 { expr }   -->   expr
+     *    (x1, ..., xn) => expr   -->   ((x1, ..., xn)) => expr
+     *       if n != 1, no alternative has a corresponding formal parameter that
+     *       is an n-ary function, and at least one alternative has a corresponding
+     *       formal parameter that is a unary function.
+     */
+    def normArg(alts: List[TermRef], arg: untpd.Tree, idx: Int): untpd.Tree = arg match
+      case Block(Nil, expr) => normArg(alts, expr, idx)
+      case untpd.Function(args: List[untpd.ValDef] @unchecked, body) =>
+
+        // If ref refers to a method whose parameter at index `idx` is a function type,
+        // the arity of that function, otherise -1.
+        def paramCount(ref: TermRef) =
+          val formals = ref.widen.firstParamTypes
+          if formals.length > idx then
+            formals(idx) match
+              case defn.FunctionOf(args, _, _, _) => args.length
+              case _ => -1
+          else -1
+
+        val numArgs = args.length
+        if numArgs != 1
+           && !alts.exists(paramCount(_) == numArgs)
+           && alts.exists(paramCount(_) == 1)
+        then
+          desugar.makeTupledFunction(args, body, isGenericTuple = true)
+            // `isGenericTuple = true` is the safe choice here. It means the i'th tuple
+            // element is selected with `(i)` instead of `_i`, which gives the same code
+            // in the end, but the compilation time and the ascribed type are more involved.
+            // It also means that -Ytest-pickler -Xprint-types fails for sources exercising
+            // the idiom since after pickling the target is known, so _i is used directly.
+        else arg
+      case _ => arg
+    end normArg
+
     val candidates = pt match {
       case pt @ FunProto(args, resultType) =>
         val numArgs = args.length
-        val normArgs = args.mapConserve {
-          case Block(Nil, expr) => expr
-          case x => x
-        }
-
         def sizeFits(alt: TermRef): Boolean = alt.widen.stripPoly match {
           case tp: MethodType =>
             val ptypes = tp.paramInfos
@@ -1661,9 +1693,10 @@ trait Applications extends Compatibility {
           alts.filter(sizeFits(_))
 
         def narrowByShapes(alts: List[TermRef]): List[TermRef] =
-          if (normArgs exists untpd.isFunctionWithUnknownParamType)
-            if (hasNamedArg(args)) narrowByTrees(alts, args map treeShape, resultType)
-            else narrowByTypes(alts, normArgs map typeShape, resultType)
+          val normArgs = args.mapWithIndexConserve(normArg(alts, _, _))
+          if normArgs.exists(untpd.isFunctionWithUnknownParamType) then
+            if hasNamedArg(args) then narrowByTrees(alts, normArgs.map(treeShape), resultType)
+            else narrowByTypes(alts, normArgs.map(typeShape), resultType)
           else
             alts
 
@@ -1681,16 +1714,14 @@ trait Applications extends Compatibility {
 
         val alts1 = narrowBySize(alts)
         //ctx.log(i"narrowed by size: ${alts1.map(_.symbol.showDcl)}%, %")
-        if (isDetermined(alts1)) alts1
-        else {
+        if isDetermined(alts1) then alts1
+        else
           val alts2 = narrowByShapes(alts1)
           //ctx.log(i"narrowed by shape: ${alts2.map(_.symbol.showDcl)}%, %")
-          if (isDetermined(alts2)) alts2
-          else {
+          if isDetermined(alts2) then alts2
+          else
             pretypeArgs(alts2, pt)
-            narrowByTrees(alts2, pt.unforcedTypedArgs, resultType)
-          }
-        }
+            narrowByTrees(alts2, pt.typedArgs(normArg(alts2, _, _)), resultType)
 
       case pt @ PolyProto(targs1, pt1) if targs.isEmpty =>
         val alts1 = alts.filter(pt.isMatchedBy(_))
@@ -1749,7 +1780,7 @@ trait Applications extends Compatibility {
     else pt match {
       case pt @ FunProto(_, resType: FunProto) =>
         // try to narrow further with snd argument list
-        val advanced = advanceCandidates(pt.unforcedTypedArgs.tpes)
+        val advanced = advanceCandidates(pt.typedArgs().tpes)
         resolveOverloaded(advanced.map(_._1), resType, Nil) // resolve with candidates where first params are stripped
           .map(advanced.toMap) // map surviving result(s) back to original candidates
       case _ =>

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

@@ -55,7 +55,7 @@ object ErrorReporting {
           case _: WildcardType | _: IgnoredProto => ""
           case tp => em" and expected result type $tp"
         }
-        em"arguments (${tp.unforcedTypedArgs.tpes}%, %)$result"
+        em"arguments (${tp.typedArgs().tpes}%, %)$result"
       case _ =>
         em"expected type $tp"
     }

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

@@ -250,7 +250,7 @@ object ProtoTypes {
     override def resultType(implicit ctx: Context): Type = resType
 
     def isMatchedBy(tp: Type, keepConstraint: Boolean)(implicit ctx: Context): Boolean = {
-      val args = unforcedTypedArgs
+      val args = typedArgs()
       def isPoly(tree: Tree) = tree.tpe.widenSingleton.isInstanceOf[PolyType]
       // See remark in normalizedCompatible for why we can't keep the constraint
       // if one of the arguments has a PolyType.
@@ -301,15 +301,18 @@ object ProtoTypes {
      *  However, any constraint changes are also propagated to the currently passed
      *  context.
      *
+     *  @param norm   a normalization function that is applied to an untyped argument tree
+     *                before it is typed. The second Int parameter is the parameter index.
      */
-    def unforcedTypedArgs(implicit ctx: Context): List[Tree] =
+    def typedArgs(norm: (untpd.Tree, Int) => untpd.Tree = sameTree)(implicit ctx: Context): List[Tree] =
       if (state.typedArgs.size == args.length) state.typedArgs
       else {
         val prevConstraint = this.ctx.typerState.constraint
 
         try {
           implicit val ctx = this.ctx
-          val args1 = args.mapconserve(cacheTypedArg(_, typer.typed(_), force = false))
+          val args1 = args.mapWithIndexConserve((arg, idx) =>
+            cacheTypedArg(arg, arg => typer.typed(norm(arg, idx)), force = false))
           if (!args1.exists(arg => isUndefined(arg.tpe))) state.typedArgs = args1
           args1
         }
@@ -375,7 +378,7 @@ object ProtoTypes {
       derivedFunProto(args, tm(resultType), typer)
 
     def fold[T](x: T, ta: TypeAccumulator[T])(implicit ctx: Context): T =
-      ta(ta.foldOver(x, unforcedTypedArgs.tpes), resultType)
+      ta(ta.foldOver(x, typedArgs().tpes), resultType)
 
     override def deepenProto(implicit ctx: Context): FunProto = derivedFunProto(args, resultType.deepenProto, typer)
 
@@ -389,7 +392,7 @@ object ProtoTypes {
    *  [](args): resultType, where args are known to be typed
    */
   class FunProtoTyped(args: List[tpd.Tree], resultType: Type)(typer: Typer, isGivenApply: Boolean)(implicit ctx: Context) extends FunProto(args, resultType)(typer, isGivenApply)(ctx) {
-    override def unforcedTypedArgs(implicit ctx: Context): List[tpd.Tree] = args
+    override def typedArgs(norm: (untpd.Tree, Int) => untpd.Tree)(implicit ctx: Context): List[tpd.Tree] = args
     override def withContext(ctx: Context): FunProtoTyped = this
   }
 
@@ -682,4 +685,6 @@ object ProtoTypes {
       case _ => None
     }
   }
+
+  private val sameTree = (t: untpd.Tree, n: Int) => t
 }

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

@@ -947,7 +947,7 @@ class Typer extends Namer
   }
 
   def typedFunctionValue(tree: untpd.Function, pt: Type)(implicit ctx: Context): Tree = {
-    val untpd.Function(params: List[untpd.ValDef] @unchecked, body) = tree
+    val untpd.Function(params: List[untpd.ValDef] @unchecked, _) = tree
 
     val isContextual = tree match {
       case tree: untpd.FunctionWithMods => tree.mods.is(Given)

compiler/test/dotc/pos-test-pickling.blacklist

@@ -32,3 +32,6 @@ i7580.scala
 
 # Nullability
 nullable.scala
+
+# parameter untupling with overloaded functions (see comment in Applications.normArg)
+i7757.scala

tests/pos/i7757.scala

@@ -0,0 +1,10 @@
+val m: Map[Int, String] = ???
+val _ = m.map((a, b) => a + b.length)
+
+trait Foo
+  def g(f: ((Int, Int)) => Int): Int = 1
+  def g(f: ((Int, Int)) => (Int, Int)): String = "2"
+
+@main def Test =
+  val m: Foo = ???
+  m.g((x: Int, b: Int) => (x, x))