Expr.betaReduce support for polymorphic function

Fixes #15968

compiler/src/scala/quoted/runtime/impl/QuotesImpl.scala

@@ -362,11 +362,14 @@ class QuotesImpl private (using val ctx: Context) extends Quotes, QuoteUnpickler
     object Term extends TermModule:
       def betaReduce(tree: Term): Option[Term] =
         tree match
-          // TODO support TypeApply. Would fix #15968.
           case app @ tpd.Apply(tpd.Select(fn, nme.apply), args) if dotc.core.Symbols.defn.isFunctionType(fn.tpe) =>
             val app1 = dotc.transform.BetaReduce(app, fn, List(args))
             if app1 eq app then None
             else Some(app1.withSpan(tree.span))
+          case app @ tpd.Apply(tpd.TypeApply(tpd.Select(fn, nme.apply), targs), args) if fn.tpe.typeSymbol eq dotc.core.Symbols.defn.PolyFunctionClass =>
+            val app1 = dotc.transform.BetaReduce(app, fn, List(targs, app.args))
+            if app1 eq app then None
+            else Some(app1.withSpan(tree.span))
           case tpd.Block(Nil, expr) =>
             for e <- betaReduce(expr) yield tpd.cpy.Block(tree)(Nil, e)
           case tpd.Inlined(_, Nil, expr) =>

tests/run-macros/i15968/Macro_1.scala

@@ -0,0 +1,15 @@
+import scala.quoted.*
+
+inline def macroPolyFun[A](inline arg: A, inline f: [Z] => Z => String): String =
+  ${ macroPolyFunImpl[A]('arg, 'f) }
+
+private def macroPolyFunImpl[A: Type](arg: Expr[A], f: Expr[[Z] => Z => String])(using Quotes): Expr[String] =
+  Expr(Expr.betaReduce('{ $f($arg) }).show)
+
+
+inline def macroFun[A](inline arg: A, inline f: A => String): String =
+  ${ macroFunImpl[A]('arg, 'f) }
+
+private def macroFunImpl[A: Type](arg: Expr[A], f: Expr[A => String])(using Quotes): Expr[String] =
+  Expr(Expr.betaReduce('{ $f($arg) }).show)
+

tests/run-macros/i15968/Test_2.scala

@@ -0,0 +1,3 @@
+@main def Test: Unit =
+  println(macroPolyFun("foo", [Z] => (arg: Z) => arg.toString))
+  println(macroFun("foo", arg => arg.toString))