Allow autotupling if fn's param is a type param

Author: dwijnand

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

@@ -2237,7 +2237,10 @@ trait Applications extends Compatibility {
 
         def isCorrectUnaryFunction(alt: TermRef): Boolean =
           val formals = params(alt)
-          formals.length == 1 && ptIsCorrectProduct(formals.head, args)
+          formals.lengthIs == 1 && {
+            ptIsCorrectProduct(formals.head, args)
+            | formals.head.dealias.isInstanceOf[TypeParamRef] // eg. i21682
+          }
 
         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) =>
+    ???