Calm param autotupling for overloads (#21552)

When resolving method overloads, we look to apply the same parameter
auto-tupling logic that we have in typedFunctionValue.  But we only
checked the function was unary without checking whether it was a tuple.
So I reused the same precondition.

Fixes #16108

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

@@ -2214,19 +2214,26 @@ trait Applications extends Compatibility {
       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) =
+        // the parameters of that function, otherwise Nil.
+        // We return Nil for both nilary functions and non-functions,
+        // because we won't be making tupled functions for nilary functions anyways,
+        // seeing as there is no Tuple0.
+        def params(ref: TermRef) =
           val formals = ref.widen.firstParamTypes
           if formals.length > idx then
             formals(idx).dealias match
-              case defn.FunctionNOf(args, _, _) => args.length
-              case _ => -1
-          else -1
+              case defn.FunctionNOf(args, _, _) => args
+              case _ => Nil
+          else Nil
+
+        def isCorrectUnaryFunction(alt: TermRef): Boolean =
+          val formals = params(alt)
+          formals.length == 1 && ptIsCorrectProduct(formals.head, args)
 
         val numArgs = args.length
-        if numArgs != 1
-           && !alts.exists(paramCount(_) == numArgs)
-           && alts.exists(paramCount(_) == 1)
+        if numArgs > 1
+           && !alts.exists(params(_).lengthIs == numArgs)
+           && alts.exists(isCorrectUnaryFunction)
         then
           desugar.makeTupledFunction(args, body, isGenericTuple = true)
             // `isGenericTuple = true` is the safe choice here. It means the i'th tuple
@@ -2395,6 +2402,13 @@ trait Applications extends Compatibility {
     }
   end resolveOverloaded1
 
+  /** Is `formal` a product type which is elementwise compatible with `params`? */
+  def ptIsCorrectProduct(formal: Type, params: List[untpd.ValDef])(using Context): Boolean =
+    isFullyDefined(formal, ForceDegree.flipBottom)
+    && defn.isProductSubType(formal)
+    && tupleComponentTypes(formal).corresponds(params): (argType, param) =>
+         param.tpt.isEmpty || argType.widenExpr <:< typedAheadType(param.tpt).tpe
+
   /** The largest suffix of `paramss` that has the same first parameter name as `t`,
    *  plus the number of term parameters in `paramss` that come before that suffix.
    */

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

@@ -1837,19 +1837,10 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
       if (protoFormals.length == params.length) (protoFormals(i), isDefinedErased(i))
       else (errorType(WrongNumberOfParameters(tree, params.length, pt, protoFormals.length), tree.srcPos), false)
 
-    /** Is `formal` a product type which is elementwise compatible with `params`? */
-    def ptIsCorrectProduct(formal: Type) =
-      isFullyDefined(formal, ForceDegree.flipBottom) &&
-      defn.isProductSubType(formal) &&
-      tupleComponentTypes(formal).corresponds(params) {
-        (argType, param) =>
-          param.tpt.isEmpty || argType.widenExpr <:< typedAheadType(param.tpt).tpe
-      }
-
     var desugared: untpd.Tree = EmptyTree
     if protoFormals.length == 1 && params.length != 1 then
       val firstFormal = protoFormals.head.loBound
-      if ptIsCorrectProduct(firstFormal) then
+      if ptIsCorrectProduct(firstFormal, params) then
         val isGenericTuple =
           firstFormal.derivesFrom(defn.TupleClass)
           && !defn.isTupleClass(firstFormal.typeSymbol)

tests/run/i16108.scala

@@ -0,0 +1,41 @@
+import scala.language.implicitConversions
+
+final class Functoid[+R](val function: Product => R)
+
+object Functoid {
+  implicit def apply[A, R](function: A => R): Functoid[R] = {
+    println(s"arity 1")
+    new Functoid({ case Tuple1(a: A @unchecked) => function(a) })
+  }
+  implicit def apply[A, B, R](function: (A, B) => R): Functoid[R] = {
+    println("arity 2")
+    new Functoid({ case (a: A @unchecked, b: B @unchecked) => function(a, b) })
+  }
+}
+
+final case class ContainerConfig(image: String, version: Int, cmd: String)
+
+final class ContainerResource
+
+object ContainerResource {
+  implicit final class DockerProviderExtensions(private val self: Functoid[ContainerResource]) extends AnyVal {
+    def modifyConfig(modify: Functoid[ContainerConfig => ContainerConfig]): Functoid[ContainerConfig => ContainerConfig] = modify
+    // removing this overload fixes the implicit conversion and returns `arity 2` print
+    def modifyConfig(modify: ContainerConfig => ContainerConfig): Functoid[ContainerConfig => ContainerConfig] = new Functoid(_ => modify)
+  }
+}
+
+object Test {
+  def main(args: Array[String]): Unit = {
+    val cfg = new Functoid(_ => new ContainerResource)
+      .modifyConfig {
+        // applying Functoid.apply explicitly instead of via implicit conversion also avoids untupling
+//        Functoid {
+        (image: String, version: Int) => (cfg: ContainerConfig) => cfg.copy(image, version)
+//        }
+      }
+      .function.apply(Tuple2("img", 9))
+      .apply(ContainerConfig("a", 0, "b"))
+    println(cfg)
+  }
+}