Allow to beta reduce curried function applications in quotes reflect

Previously, the curried functions with multiple applications
were not able to be beta-reduced in any way, which was unexpected.
Now we allow reducing any number of top-level function applications
for a curried function. This was also made clearer in the documentation
for the affected (Expr.betaReduce and Term.betaReduce) methods.

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

@@ -396,17 +396,22 @@ class QuotesImpl private (using val ctx: Context) extends Quotes, QuoteUnpickler
     end TermTypeTest
 
     object Term extends TermModule:
-      def betaReduce(tree: Term): Option[Term] =
-        tree match
-          case tpd.Block(Nil, expr) =>
-            for e <- betaReduce(expr) yield tpd.cpy.Block(tree)(Nil, e)
-          case tpd.Inlined(_, Nil, expr) =>
-            betaReduce(expr)
-          case _ =>
-            val tree1 = dotc.transform.BetaReduce(tree)
-            if tree1 eq tree then None
-            else Some(tree1.withSpan(tree.span))
-
+       def betaReduce(tree: Term): Option[Term] =
+        val tree1 = new dotty.tools.dotc.ast.tpd.TreeMap {
+          override def transform(tree: Tree)(using Context): Tree = tree match {
+            case tpd.Block(Nil, _) |  tpd.Inlined(_, Nil, _) =>
+              super.transform(tree)
+            case tpd.Apply(sel @ tpd.Select(expr, nme), args) =>
+              val tree1 = cpy.Apply(tree)(cpy.Select(sel)(transform(expr), nme), args)
+              dotc.transform.BetaReduce(tree1).withSpan(tree.span)
+            case tpd.Apply(ta @ tpd.TypeApply(sel @ tpd.Select(expr: Apply, nme), tpts), args) =>
+              val tree1 = cpy.Apply(tree)(cpy.TypeApply(ta)(cpy.Select(sel)(transform(expr), nme), tpts), args)
+              dotc.transform.BetaReduce(tree1).withSpan(tree.span)
+            case _ =>
+              dotc.transform.BetaReduce(tree).withSpan(tree.span)
+          }
+        }.transform(tree)
+        if tree1 == tree then None else Some(tree1)
     end Term
 
     given TermMethods: TermMethods with

library/src/scala/quoted/Expr.scala

@@ -10,12 +10,45 @@ abstract class Expr[+T] private[scala] ()
 object Expr {
 
   /** `e.betaReduce` returns an expression that is functionally equivalent to `e`,
-   *   however if `e` is of the form `((y1, ..., yn) => e2)(e1, ..., en)`
-   *   then it optimizes this the top most call by returning the result of beta-reducing the application.
-   *   Otherwise returns `expr`.
+   *  however if `e` is of the form `((y1, ..., yn) => e2)(e1, ..., en)`
+   *  then it optimizes the top most call by returning the result of beta-reducing the application.
+   *  Similarly, all outermost curried function applications will be beta-reduced, if possible.
+   *  Otherwise returns `expr`.
    *
-   *   To retain semantics the argument `ei` is bound as `val yi = ei` and by-name arguments to `def yi = ei`.
-   *   Some bindings may be elided as an early optimization.
+   *  To retain semantics the argument `ei` is bound as `val yi = ei` and by-name arguments to `def yi = ei`.
+   *  Some bindings may be elided as an early optimization.
+   *
+   *  Example:
+   *  ```scala sc:nocompile
+   *  ((a: Int, b: Int) => a + b).apply(x, y)
+   *  ```
+   *  will be reduced to
+   *  ```scala sc:nocompile
+   *  val a = x
+   *  val b = y
+   *  a + b
+   *  ```
+   *
+   *  Generally:
+   *  ```scala sc:nocompile
+   *  ([X1, Y1, ...] => (x1, y1, ...) => ... => [Xn, Yn, ...] => (xn, yn, ...) => f[X1, Y1, ..., Xn, Yn, ...](x1, y1, ..., xn, yn, ...))).apply[Tx1, Ty1, ...](myX1, myY1, ...)....apply[Txn, Tyn, ...](myXn, myYn, ...)
+   *  ```
+   *  will be reduced to
+   *  ```scala sc:nocompile
+   *  type X1 = Tx1
+   *  type Y1 = Ty1
+   *  ...
+   *  val x1 = myX1
+   *  val y1 = myY1
+   *  ...
+   *  type Xn = Txn
+   *  type Yn = Tyn
+   *  ...
+   *  val xn = myXn
+   *  val yn = myYn
+   *  ...
+   *  f[X1, Y1, ..., Xn, Yn, ...](x1, y1, ..., xn, yn, ...)
+   *  ```
    */
   def betaReduce[T](expr: Expr[T])(using Quotes): Expr[T] =
     import quotes.reflect.*

library/src/scala/quoted/Quotes.scala

@@ -774,14 +774,47 @@ trait Quotes { self: runtime.QuoteUnpickler & runtime.QuoteMatching =>
     /** Methods of the module object `val Term` */
     trait TermModule { this: Term.type =>
 
-      /** Returns a term that is functionally equivalent to `t`,
+     /** Returns a term that is functionally equivalent to `t`,
       *  however if `t` is of the form `((y1, ..., yn) => e2)(e1, ..., en)`
-      *  then it optimizes this the top most call by returning the `Some`
-      *  with the result of beta-reducing the application.
+      *  then it optimizes the top most call by returning `Some`
+      *  with the result of beta-reducing the function application.
+      *  Similarly, all outermost curried function applications will be beta-reduced, if possible.
       *  Otherwise returns `None`.
       *
-      *   To retain semantics the argument `ei` is bound as `val yi = ei` and by-name arguments to `def yi = ei`.
-      *   Some bindings may be elided as an early optimization.
+      *  To retain semantics the argument `ei` is bound as `val yi = ei` and by-name arguments to `def yi = ei`.
+      *  Some bindings may be elided as an early optimization.
+      *
+      *  Example:
+      *  ```scala sc:nocompile
+      *  ((a: Int, b: Int) => a + b).apply(x, y)
+      *  ```
+      *  will be reduced to
+      *  ```scala sc:nocompile
+      *  val a = x
+      *  val b = y
+      *  a + b
+      *  ```
+      *
+      *  Generally:
+      *  ```scala sc:nocompile
+      *  ([X1, Y1, ...] => (x1, y1, ...) => ... => [Xn, Yn, ...] => (xn, yn, ...) => f[X1, Y1, ..., Xn, Yn, ...](x1, y1, ..., xn, yn, ...))).apply[Tx1, Ty1, ...](myX1, myY1, ...)....apply[Txn, Tyn, ...](myXn, myYn, ...)
+      *  ```
+      *  will be reduced to
+      *  ```scala sc:nocompile
+      *  type X1 = Tx1
+      *  type Y1 = Ty1
+      *  ...
+      *  val x1 = myX1
+      *  val y1 = myY1
+      *  ...
+      *  type Xn = Txn
+      *  type Yn = Tyn
+      *  ...
+      *  val xn = myXn
+      *  val yn = myYn
+      *  ...
+      *  f[X1, Y1, ..., Xn, Yn, ...](x1, y1, ..., xn, yn, ...)
+      *  ```
       */
       def betaReduce(term: Term): Option[Term]
 

tests/pos-macros/i17506/Macro_1.scala

@@ -0,0 +1,80 @@
+class Foo
+class Bar
+class Baz
+
+import scala.quoted._
+
+def assertBetaReduction(using Quotes)(applied: Expr[Any], expected: String): quotes.reflect.Term =
+  import quotes.reflect._
+  val reducedMaybe = Term.betaReduce(applied.asTerm)
+  assert(reducedMaybe.isDefined)
+  val reduced = reducedMaybe.get
+  assert(reduced.show == expected,s"obtained: ${reduced.show}, expected: ${expected}")
+  reduced
+
+inline def regularCurriedCtxFun2BetaReduceTest(inline f: Foo ?=> Bar ?=> Int): Unit =
+  ${regularCurriedCtxFun2BetaReduceTestImpl('f)}
+def regularCurriedCtxFun2BetaReduceTestImpl(f: Expr[Foo ?=> Bar ?=> Int])(using Quotes): Expr[Int] =
+  val expected =
+    """|{
+       |  val contextual$3: Bar = new Bar()
+       |  val contextual$2: Foo = new Foo()
+       |  123
+       |}""".stripMargin
+  val applied = '{$f(using new Foo())(using new Bar())}
+  assertBetaReduction(applied, expected).asExprOf[Int]
+
+inline def regularCurriedFun2BetaReduceTest(inline f: Foo => Bar => Int): Int =
+  ${regularCurriedFun2BetaReduceTestImpl('f)}
+def regularCurriedFun2BetaReduceTestImpl(f: Expr[Foo => Bar => Int])(using Quotes): Expr[Int] =
+  val expected =
+    """|{
+       |  val b: Bar = new Bar()
+       |  val f: Foo = new Foo()
+       |  123
+       |}""".stripMargin
+  val applied = '{$f(new Foo())(new Bar())}
+  assertBetaReduction(applied, expected).asExprOf[Int]
+
+inline def typeParamCurriedFun2BetaReduceTest(inline f: [A] => A => [B] => B => Unit): Unit =
+  ${typeParamCurriedFun2BetaReduceTestImpl('f)}
+def typeParamCurriedFun2BetaReduceTestImpl(f: Expr[[A] => (a: A) => [B] => (b: B) => Unit])(using Quotes): Expr[Unit] =
+  val expected =
+    """|{
+       |  type Y = Bar
+       |  val y: Bar = new Bar()
+       |  type X = Foo
+       |  val x: Foo = new Foo()
+       |  typeParamFun2[Y, X](y, x)
+       |}""".stripMargin
+  val applied = '{$f.apply[Foo](new Foo()).apply[Bar](new Bar())}
+  assertBetaReduction(applied, expected).asExprOf[Unit]
+
+inline def regularCurriedFun3BetaReduceTest(inline f: Foo => Bar => Baz => Int): Int =
+  ${regularCurriedFun3BetaReduceTestImpl('f)}
+def regularCurriedFun3BetaReduceTestImpl(f: Expr[Foo => Bar => Baz => Int])(using Quotes): Expr[Int] =
+  val expected =
+    """|{
+       |  val i: Baz = new Baz()
+       |  val b: Bar = new Bar()
+       |  val f: Foo = new Foo()
+       |  123
+       |}""".stripMargin
+  val applied = '{$f(new Foo())(new Bar())(new Baz())}
+  assertBetaReduction(applied, expected).asExprOf[Int]
+
+inline def typeParamCurriedFun3BetaReduceTest(inline f: [A] => A => [B] => B => [C] => C => Unit): Unit =
+  ${typeParamCurriedFun3BetaReduceTestImpl('f)}
+def typeParamCurriedFun3BetaReduceTestImpl(f: Expr[[A] => A => [B] => B => [C] => C => Unit])(using Quotes): Expr[Unit] =
+  val expected =
+    """|{
+       |  type Z = Baz
+       |  val z: Baz = new Baz()
+       |  type Y = Bar
+       |  val y: Bar = new Bar()
+       |  type X = Foo
+       |  val x: Foo = new Foo()
+       |  typeParamFun3[Z, Y, X](z, y, x)
+       |}""".stripMargin
+  val applied = '{$f.apply[Foo](new Foo()).apply[Bar](new Bar()).apply[Baz](new Baz())}
+  assertBetaReduction(applied, expected).asExprOf[Unit]

tests/pos-macros/i17506/Test_2.scala

@@ -0,0 +1,11 @@
+@main def run() =
+  def typeParamFun2[A, B](a: A, b: B): Unit = println(a.toString + " " + b.toString)
+  def typeParamFun3[A, B, C](a: A, b: B, c: C): Unit = println(a.toString + " " + b.toString)
+
+  regularCurriedCtxFun2BetaReduceTest((f: Foo) ?=> (b: Bar) ?=> 123)
+  regularCurriedCtxFun2BetaReduceTest(123)
+  regularCurriedFun2BetaReduceTest(((f: Foo) => (b: Bar) => 123))
+  typeParamCurriedFun2BetaReduceTest([X] => (x: X) => [Y] => (y: Y) => typeParamFun2[Y, X](y, x))
+
+  regularCurriedFun3BetaReduceTest((f: Foo) => (b: Bar) => (i: Baz) => 123)
+  typeParamCurriedFun3BetaReduceTest([X] => (x: X) => [Y] => (y: Y) => [Z] => (z: Z) => typeParamFun3[Z, Y, X](z, y, x))