Redefine quoted.Expr.betaReduce

Redefine `quoted.Expr.betaRduce` to not rely on complex type level computations.
Changed the signature as follows
```diff
- def betaReduce[F, Args <: Tuple, R, G](f: Expr[F])(using tf: TupledFunction[F, Args => R], tg: TupledFunction[G, TupleOfExpr[Args] => Expr[R]], qctx: QuoteContext): G = ...
+ def betaReduce[T](expr: Expr[T])(using qctx: QuoteContext): Option[Expr[T]] = ...
```

Improvements
* Simpler API that covers all kind of functions at once (normal/given/erased)
* Better error message for ill-typed `betaRduce` calls
* Adds the possiblility of knowing if the beta-reeduction suceeded
* Use `transform.BetaReduce`
* Improve `transform.BetaReduce` to handle `Inlined` trees and constant argumets
* Fixes #9466

Drawback
* Need for slightly loneger code (previous interface could be implented on top of this one)

compiler/src/dotty/tools/dotc/tastyreflect/ReflectionCompilerInterface.scala

@@ -2042,21 +2042,18 @@ class ReflectionCompilerInterface(val rootContext: core.Contexts.Context) extend
       case _ => None
   }
 
-  def betaReduce(fn: Term, args: List[Term])(using Context): Term = {
-    val (argVals0, argRefs0) = args.foldLeft((List.empty[ValDef], List.empty[Tree])) { case ((acc1, acc2), arg) => arg.tpe match {
-      case tpe: SingletonType if isIdempotentExpr(arg) => (acc1, arg :: acc2)
+  def betaReduce(tree: Term)(using Context): Option[Term] =
+    tree match
+      case app @ Apply(Select(fn, nme.apply), args) if defn.isFunctionType(fn.tpe) =>
+        val app1 = transform.BetaReduce(app, fn, args)
+        if app1 eq app then None
+        else Some(app1.withSpan(tree.span))
+      case Block(Nil, expr) =>
+        for e <- betaReduce(expr) yield cpy.Block(tree)(Nil, e)
+      case Inlined(_, Nil, expr) =>
+        betaReduce(expr)
       case _ =>
-        val argVal = SyntheticValDef(NameKinds.UniqueName.fresh("x".toTermName), arg).withSpan(arg.span)
-        (argVal :: acc1, ref(argVal.symbol) :: acc2)
-    }}
-    val argVals = argVals0.reverse
-    val argRefs = argRefs0.reverse
-    val reducedBody = lambdaExtractor(fn, argRefs.map(_.tpe)) match {
-      case Some(body) => body(argRefs)
-      case None => fn.select(nme.apply).appliedToArgs(argRefs)
-    }
-    seq(argVals, reducedBody).withSpan(fn.span)
-  }
+        None
 
   def lambdaExtractor(fn: Term, paramTypes: List[Type])(using Context): Option[List[Term] => Term] = {
     def rec(fn: Term, transformBody: Term => Term): Option[List[Term] => Term] = {

compiler/src/dotty/tools/dotc/transform/BetaReduce.scala

@@ -37,22 +37,26 @@ class BetaReduce extends MiniPhase:
 
   override def transformApply(app: Apply)(using Context): Tree = app.fun match
     case Select(fn, nme.apply) if defn.isFunctionType(fn.tpe) =>
-      val app1 = betaReduce(app, fn, app.args)
+      val app1 = BetaReduce(app, fn, app.args)
       if app1 ne app then report.log(i"beta reduce $app -> $app1")
       app1
     case _ =>
       app
 
-  private def betaReduce(tree: Apply, fn: Tree, args: List[Tree])(using Context): Tree =
-    fn match
-      case Typed(expr, _) => betaReduce(tree, expr, args)
-      case Block(Nil, expr) => betaReduce(tree, expr, args)
-      case Block((anonFun: DefDef) :: Nil, closure: Closure) => BetaReduce(anonFun, args)
-      case _ => tree
 
 object BetaReduce:
   import ast.tpd._
 
+  /** Beta-reduces a call to `fn` with arguments `argSyms` or returns `tree` */
+  def apply(tree: Apply, fn: Tree, args: List[Tree])(using Context): Tree =
+    fn match
+      case Typed(expr, _) => BetaReduce(tree, expr, args)
+      case Block(Nil, expr) => BetaReduce(tree, expr, args)
+      case Inlined(_, Nil, expr) => BetaReduce(tree, expr, args)
+      case Block((anonFun: DefDef) :: Nil, closure: Closure) => BetaReduce(anonFun, args)
+      case _ => tree
+  end apply
+
   /** Beta-reduces a call to `ddef` with arguments `argSyms` */
   def apply(ddef: DefDef, args: List[Tree])(using Context) =
     val bindings = List.newBuilder[ValDef]
@@ -65,7 +69,8 @@ object BetaReduce:
             ref.symbol
           case _ =>
             val flags = Synthetic | (param.symbol.flags & Erased)
-            val binding = ValDef(newSymbol(ctx.owner, param.name, flags, arg.tpe.widen, coord = arg.span), arg)
+            val tpe = if arg.tpe.dealias.isInstanceOf[ConstantType] then arg.tpe.dealias else arg.tpe.widen
+            val binding = ValDef(newSymbol(ctx.owner, param.name, flags, tpe, coord = arg.span), arg).withSpan(arg.span)
             bindings += binding
             binding.symbol
 
@@ -76,5 +81,13 @@ object BetaReduce:
       substTo = argSyms
     ).transform(ddef.rhs)
 
-    seq(bindings.result(), expansion)
+    val expansion1 = new TreeMap {
+      override def transform(tree: Tree)(using Context) = tree.tpe.widenTermRefExpr match
+        case ConstantType(const) if isPureExpr(tree) => cpy.Literal(tree)(const)
+        case _ => super.transform(tree)
+    }.transform(expansion)
+    val bindings1 =
+      bindings.result().filterNot(vdef => vdef.tpt.tpe.isInstanceOf[ConstantType] && isPureExpr(vdef.rhs))
+
+    seq(bindings1, expansion1)
   end apply

compiler/test/dotty/tools/backend/jvm/InlineBytecodeTests.scala

@@ -399,22 +399,15 @@ class InlineBytecodeTests extends DottyBytecodeTest {
       val instructions = instructionsFromMethod(fun)
       val expected =
         List(
-          // Head tested separatly
           VarOp(ALOAD, 0),
           Invoke(INVOKEVIRTUAL, "Test", "given_Int", "()I", false),
-          Invoke(INVOKESTATIC, "scala/runtime/BoxesRunTime", "boxToInteger", "(I)Ljava/lang/Integer;", false),
-          Invoke(INVOKEINTERFACE, "dotty/runtime/function/JFunction1$mcZI$sp", "apply", "(Ljava/lang/Object;)Ljava/lang/Object;", true),
-          Invoke(INVOKESTATIC, "scala/runtime/BoxesRunTime", "unboxToBoolean", "(Ljava/lang/Object;)Z", false),
+          VarOp(ISTORE, 1),
+          Op(ICONST_1),
           Op(IRETURN)
         )
 
-      instructions.head match {
-        case InvokeDynamic(INVOKEDYNAMIC, "apply$mcZI$sp", "()Ldotty/runtime/function/JFunction1$mcZI$sp;", _, _) =>
-        case _ => assert(false, "`g` was not properly inlined in `test`\n")
-      }
-
-      assert(instructions.tail == expected,
-        "`fg was not properly inlined in `test`\n" + diffInstructions(instructions.tail, expected))
+      assert(instructions == expected,
+        "`fg was not properly inlined in `test`\n" + diffInstructions(instructions, expected))
 
     }
   }
@@ -505,4 +498,30 @@ class InlineBytecodeTests extends DottyBytecodeTest {
     }
   }
 
+
+  @Test def i9466 = {
+    val source = """class Test:
+                   |  inline def i(inline f: Int => Boolean): String =
+                   |   if f(34) then "a"
+                   |   else "b"
+                   |  def test = i(f = _ == 34)
+                 """.stripMargin
+
+    checkBCode(source) { dir =>
+      val clsIn      = dir.lookupName("Test.class", directory = false).input
+      val clsNode    = loadClassNode(clsIn)
+
+      val fun = getMethod(clsNode, "test")
+      val instructions = instructionsFromMethod(fun)
+      val expected =
+        List(
+          Ldc(LDC, "a"),
+          Op(ARETURN)
+        )
+
+      assert(instructions == expected,
+        "`i was not properly inlined in `test`\n" + diffInstructions(instructions, expected))
+
+    }
+  }
 }

library/src-bootstrapped/scala/quoted/Expr.scala

@@ -61,30 +61,13 @@ abstract class Expr[+T] private[scala] {
 
 object Expr {
 
-  /** Converts a tuple `(T1, ..., Tn)` to `(Expr[T1], ..., Expr[Tn])` */
-  type TupleOfExpr[Tup <: Tuple] = Tuple.Map[Tup, [X] =>> QuoteContext ?=> Expr[X]]
-
-  /** `Expr.betaReduce(f)(x1, ..., xn)` is functionally the same as `'{($f)($x1, ..., $xn)}`, however it optimizes this call
-   *   by returning the result of beta-reducing `f(x1, ..., xn)` if `f` is a known lambda expression.
-   *
-   *  `Expr.betaReduce` distributes applications of `Expr` over function arrows
-   *   ```scala
-   *   Expr.betaReduce(_): Expr[(T1, ..., Tn) => R] => ((Expr[T1], ..., Expr[Tn]) => Expr[R])
-   *   ```
-   */
-  def betaReduce[F, Args <: Tuple, R, G](f: Expr[F])(using tf: TupledFunction[F, Args => R], tg: TupledFunction[G, TupleOfExpr[Args] => Expr[R]], qctx: QuoteContext): G =
-    tg.untupled(args => qctx.tasty.internal.betaReduce(f.unseal, args.toArray.toList.map(_.asInstanceOf[QuoteContext => Expr[Any]](qctx).unseal)).seal.asInstanceOf[Expr[R]])
-
-  /** `Expr.betaReduceGiven(f)(x1, ..., xn)` is functionally the same as `'{($f)(using $x1, ..., $xn)}`, however it optimizes this call
-   *   by returning the result of beta-reducing `f(using x1, ..., xn)` if `f` is a known lambda expression.
-   *
-   *  `Expr.betaReduceGiven` distributes applications of `Expr` over function arrows
-   *   ```scala
-   *   Expr.betaReduceGiven(_): Expr[(T1, ..., Tn) ?=> R] => ((Expr[T1], ..., Expr[Tn]) => Expr[R])
-   *   ```
+  /** `e.betaReduce` returns a option with a expression that is functionally equivalent to `e`,
+   *   however if `e` is of the form `((y1, ..., yn) => e2)(x1, ..., xn)`
+   *   then it optimizes this the top most call by returning `Some` with the result of beta-reducing the application.
+   *   Otherwise returns None.
    */
-  def betaReduceGiven[F, Args <: Tuple, R, G](f: Expr[F])(using tf: TupledFunction[F, Args ?=> R], tg: TupledFunction[G, TupleOfExpr[Args] => Expr[R]], qctx: QuoteContext): G =
-    tg.untupled(args => qctx.tasty.internal.betaReduce(f.unseal, args.toArray.toList.map(_.asInstanceOf[QuoteContext => Expr[Any]](qctx).unseal)).seal.asInstanceOf[Expr[R]])
+  def betaReduce[T](expr: Expr[T])(using qctx: QuoteContext): Option[Expr[T]] =
+    qctx.tasty.internal.betaReduce(expr.unseal).map(_.seal.asInstanceOf[Expr[T]])
 
   /** Returns a null expresssion equivalent to `'{null}` */
   def nullExpr: QuoteContext ?=> Expr[Null] = qctx ?=> {

library/src-bootstrapped/scala/quoted/unsafe/UnsafeExpr.scala

@@ -22,7 +22,7 @@ object UnsafeExpr {
   def underlyingArgument[T](expr: Expr[T])(using qctx: QuoteContext): Expr[T] =
     expr.unseal.underlyingArgument.seal.asInstanceOf[Expr[T]]
 
-  // TODO generalize for any function arity (see Expr.betaReduce)
+  // TODO generalize for any function arity
   /** Allows inspection or transformation of the body of the expression of function.
    *  This body may have references to the arguments of the function which should be closed
    *  over if the expression will be spliced.

library/src/scala/tasty/reflect/CompilerInterface.scala

@@ -1600,10 +1600,8 @@ trait CompilerInterface {
    */
   def searchImplicit(tpe: Type)(using ctx: Context): ImplicitSearchResult
 
-  /** Inline fn if it is an explicit closure possibly nested inside the expression of a block.
-   *  Otherwise apply the arguments to the closure.
-   */
-  def betaReduce(f: Term, args: List[Term])(using ctx: Context): Term
+  /** Returns Some with a beta-reduced application or None */
+  def betaReduce(tree: Term)(using Context): Option[Term]
 
   def lambdaExtractor(term: Term, paramTypes: List[Type])(using ctx: Context): Option[List[Term] => Term]
 

tests/neg-macros/beta-reduce-inline-result/Macro_1.scala

@@ -4,7 +4,7 @@ object Macros {
   inline def betaReduce[Arg,Result](inline fn: Arg=>Result)(inline arg: Arg): Result =
     ${ betaReduceImpl('{ fn })('{ arg }) }
 
-  def betaReduceImpl[Arg,Result](fn: Expr[Arg=>Result])(arg: Expr[Arg])(using qctx: QuoteContext): Expr[Result] =
-    Expr.betaReduce(fn)(arg)
+  def betaReduceImpl[Arg: Type, Result: Type](fn: Expr[Arg=>Result])(arg: Expr[Arg])(using qctx: QuoteContext): Expr[Result] =
+    Expr.betaReduce('{$fn($arg)}).get
 }
 

tests/pos-macros/i6783.scala

@@ -1,6 +1,6 @@
 import scala.quoted._
 
-def testImpl(f: Expr[(Int, Int) => Int])(using QuoteContext): Expr[Int] = Expr.betaReduce(f)('{1}, '{2})
+def testImpl(f: Expr[(Int, Int) => Int])(using QuoteContext): Expr[Int] = Expr.betaReduce('{$f(1, 2)}).get
 
 inline def test(f: (Int, Int) => Int) = ${
   testImpl('f)

tests/run-macros/beta-reduce-inline-result.check

@@ -1,6 +1,6 @@
-compile-time: ((3.+(1): scala.Int): scala.Int)
+compile-time: (4: scala.Int)
 run-time: 4
-compile-time: ((1: 1): scala.Int)
+compile-time: (1: scala.Int)
 run-time: 1
 run-time: 5
 run-time: 7

tests/run-macros/beta-reduce-inline-result/Macro_1.scala

@@ -4,13 +4,15 @@ object Macros {
   inline def betaReduce[Arg,Result](inline fn : Arg=>Result)(inline arg: Arg): Result =
     ${ betaReduceImpl('{ fn })('{ arg }) }
 
-  def betaReduceImpl[Arg,Result](fn: Expr[Arg=>Result])(arg: Expr[Arg])(using qctx : QuoteContext): Expr[Result] =
-    Expr.betaReduce(fn)(arg)
+  def betaReduceImpl[Arg: Type, Result: Type](fn: Expr[Arg=>Result])(arg: Expr[Arg])(using qctx : QuoteContext): Expr[Result] =
+    val app = '{$fn($arg)}
+    Expr.betaReduce(app).getOrElse(app)
 
   inline def betaReduceAdd1[Arg](inline fn: Arg=>Int)(inline arg: Arg): Int =
     ${ betaReduceAdd1Impl('{ fn })('{ arg }) }
 
-  def betaReduceAdd1Impl[Arg](fn: Expr[Arg=>Int])(arg: Expr[Arg])(using qctx: QuoteContext): Expr[Int] =
-    '{ ${ Expr.betaReduce(fn)(arg) } + 1 }
+  def betaReduceAdd1Impl[Arg: Type](fn: Expr[Arg=>Int])(arg: Expr[Arg])(using qctx: QuoteContext): Expr[Int] =
+    val app = '{$fn.asInstanceOf[Arg=>Int]($arg)} // FIXME: remove asInstanceOf (workaround for #8612)
+    '{ ${ Expr.betaReduce(app).getOrElse(app) } + 1 }
 }
 

tests/run-macros/beta-reduce-inline-result/Test_2.scala

@@ -74,4 +74,3 @@ object Test {
     println(s"run-time: ${Macros.betaReduce(dummy7)(8)}")
   }
 }
-

tests/run-macros/gestalt-optional-staging/Macro_1.scala

@@ -9,7 +9,7 @@ final class Optional[+A >: Null](val value: A) extends AnyVal {
 
   inline def getOrElse[B >: A](alt: => B): B = ${ Optional.getOrElseImpl('this, 'alt) }
 
-  inline def map[B >: Null](f: A => B): Optional[B] = ${ Optional.mapImpl('this, 'f) }
+  inline def map[B >: Null](inline f: A => B): Optional[B] = ${ Optional.mapImpl('this, 'f) }
 
   override def toString = if (isEmpty) "<empty>" else s"$value"
 }
@@ -24,7 +24,7 @@ object Optional {
   // FIXME fix issue #5097 and enable private
   /*private*/ def mapImpl[A >: Null : Type, B >: Null : Type](opt: Expr[Optional[A]], f: Expr[A => B])(using QuoteContext): Expr[Optional[B]] = '{
     if ($opt.isEmpty) new Optional(null)
-    else new Optional(${Expr.betaReduce(f)('{$opt.value})})
+    else new Optional(${Expr.betaReduce('{$f($opt.value)}).get})
   }
 
 }

tests/run-macros/i4734/Macro_1.scala

@@ -2,7 +2,7 @@ import scala.annotation.tailrec
 import scala.quoted._
 
 object Macros {
-  inline def unrolledForeach(seq: IndexedSeq[Int], f: => Int => Unit, inline unrollSize: Int): Unit = // or f: Int => Unit
+  inline def unrolledForeach(seq: IndexedSeq[Int], inline f: Int => Unit, inline unrollSize: Int): Unit = // or f: Int => Unit
     ${ unrolledForeachImpl('seq, 'f, 'unrollSize) }
 
   def unrolledForeachImpl(seq: Expr[IndexedSeq[Int]], f: Expr[Int => Unit], unrollSizeExpr: Expr[Int]) (using QuoteContext): Expr[Unit] =
@@ -17,7 +17,7 @@ object Macros {
         for (j <- new UnrolledRange(0, unrollSize)) '{
           val index = i + ${Expr(j)}
           val element = ($seq)(index)
-          ${ Expr.betaReduce(f)('element) } // or `($f)(element)` if `f` should not be inlined
+          ${ Expr.betaReduce('{$f(element)}).get } // or `($f)(element)` if `f` should not be inlined
         }
       }
       i += ${Expr(unrollSize)}

tests/run-macros/i4735/App_2.scala

@@ -10,5 +10,5 @@ object Test {
 }
 
 class Unrolled(arr: Array[Int]) extends AnyVal {
-  inline def foreach(f: => Int => Unit): Unit = Macro.unrolledForeach(3, arr, f)
+  inline def foreach(inline f: Int => Unit): Unit = Macro.unrolledForeach(3, arr, f)
 }

tests/run-macros/i4735/Macro_1.scala

@@ -4,7 +4,7 @@ import scala.quoted._
 
 object Macro {
 
-  inline def unrolledForeach(inline unrollSize: Int, seq: Array[Int], f: => Int => Unit): Unit = // or f: Int => Unit
+  inline def unrolledForeach(inline unrollSize: Int, seq: Array[Int], inline f: Int => Unit): Unit = // or f: Int => Unit
     ${ unrolledForeachImpl('unrollSize, 'seq, 'f) }
 
   private def unrolledForeachImpl(unrollSize: Expr[Int], seq: Expr[Array[Int]], f: Expr[Int => Unit]) (using QuoteContext): Expr[Unit] = '{
@@ -16,7 +16,7 @@ object Macro {
       ${
         for (j <- new UnrolledRange(0, unrollSize.unliftOrError)) '{
           val element = ($seq)(i + ${Expr(j)})
-          ${Expr.betaReduce(f)('element)} // or `($f)(element)` if `f` should not be inlined
+          ${Expr.betaReduce('{$f(element)}).get} // or `($f)(element)` if `f` should not be inlined
         }
       }
       i += ${unrollSize}

tests/run-macros/i7008/macro_1.scala

@@ -13,5 +13,5 @@ def mcrProxy(expr: Expr[Boolean])(using QuoteContext): Expr[Unit] = {
 def mcrImpl[T](func: Expr[Seq[Box[T]] => Unit], expr: Expr[T])(using ctx: QuoteContext, tt: Type[T]): Expr[Unit] = {
   import ctx.tasty._
   val arg = Varargs(Seq('{(Box($expr))}))
-  Expr.betaReduce(func)(arg)
+  Expr.betaReduce('{$func($arg)}).get
 }

tests/run-macros/quote-inline-function.check

@@ -3,13 +3,11 @@ Normal function
   var i: scala.Int = 0
   val j: scala.Int = 5
   while (i.<(j)) {
-    val x$1: scala.Int = i
-    f.apply(x$1)
+    f.apply(i)
     i = i.+(1)
   }
   while ({
-    val x$2: scala.Int = i
-    f.apply(x$2)
+    f.apply(i)
     i = i.+(1)
     i.<(j)
   }) ()
@@ -20,13 +18,11 @@ By name function
   var i: scala.Int = 0
   val j: scala.Int = 5
   while (i.<(j)) {
-    val x$3: scala.Int = i
-    f.apply(x$3)
+    f.apply(i)
     i = i.+(1)
   }
   while ({
-    val x$4: scala.Int = i
-    f.apply(x$4)
+    f.apply(i)
     i = i.+(1)
     i.<(j)
   }) ()
@@ -37,13 +33,13 @@ Inline function
   var i: scala.Int = 0
   val j: scala.Int = 5
   while (i.<(j)) {
-    val x$5: scala.Int = i
-    scala.Predef.println(x$5)
+    val x: scala.Int = i
+    scala.Predef.println(x)
     i = i.+(1)
   }
   while ({
-    val x$6: scala.Int = i
-    scala.Predef.println(x$6)
+    val `x₂`: scala.Int = i
+    scala.Predef.println(`x₂`)
     i = i.+(1)
     i.<(j)
   }) ()

tests/run-macros/quote-inline-function/quoted_1.scala

@@ -12,11 +12,11 @@ object Macros {
       var i = $start
       val j = $end
       while (i < j) {
-        ${Expr.betaReduce(f)('i)}
+        ${Expr.betaReduce('{$f(i)}).getOrElse('{$f(i)})}
         i += 1
       }
       while {
-        ${Expr.betaReduce(f)('i)}
+        ${Expr.betaReduce('{$f(i)}).getOrElse('{$f(i)})}
         i += 1
         i < j
       } do ()