Allow autotupling if fn's param is a type param (#21741)

Fixes #21682

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

@@ -2237,7 +2237,19 @@ trait Applications extends Compatibility {
 
         def isCorrectUnaryFunction(alt: TermRef): Boolean =
           val formals = params(alt)
-          formals.length == 1 && ptIsCorrectProduct(formals.head, args)
+          formals.length == 1 && {
+            formals.head match
+              case formal: TypeParamRef =>
+                // While `formal` isn't a tuple type of the correct arity,
+                // it's a type parameter (a method type parameter presumably)
+                // so check its bounds allow for a tuple type of the correct arity.
+                // See i21682 for an example.
+                val tup = defn.tupleType(args.map(v => if v.tpt.isEmpty then WildcardType else typedAheadType(v.tpt).tpe))
+                val TypeBounds(lo, hi) = formal.paramInfo
+                lo <:< tup && tup <:< hi
+              case formal =>
+                ptIsCorrectProduct(formal, args)
+          }
 
         val numArgs = args.length
         if numArgs > 1

tests/pos/i21682.1.scala

@@ -0,0 +1,15 @@
+sealed abstract class Gen[+T1]
+given [T2]: Conversion[T2, Gen[T2]] = ???
+
+trait Show[T3]
+given Show[Boolean] = ???
+given [A1: Show, B1: Show, C1: Show]: Show[(A1, B1, C1)] = ???
+
+object ForAll:
+  def apply[A2: Show, B2](f: A2 => B2): Unit = ???
+  def apply[A3: Show, B3: Show, C3](f: (A3, B3) => C3): Unit = ???
+  def apply[A4: Show, B4](gen: Gen[A4])(f: A4 => B4): Unit = ???
+
+@main def Test =
+  ForAll: (b1: Boolean, b2: Boolean, b3: Boolean) =>
+    ???

tests/pos/i21682.2.scala

@@ -0,0 +1,7 @@
+object ForAll:
+  def apply[A1, B](f: A1 => B): Unit = ???
+  def apply[A1, A2, B](f: (A1, A2) => B): Unit = ???
+
+@main def Test =
+  ForAll: (b1: Boolean, b2: Boolean, b3: Boolean) =>
+    ???

tests/pos/i21682.3.scala

@@ -0,0 +1,4 @@
+class Test:
+  def foo[A1 >: (Nothing, Boolean, Nothing) <: (Any, Boolean, Any), B](f: A1 => B): Unit = ???
+  def test(): Unit =
+    val res4 = this.foo((b1: Boolean, b2: Boolean, b3: Boolean) => ???)