Attempt to beta reduce only if parameter and argument lists have same…

… shape

It's possible to define Functions with wrong apply methods by hand which will
give an error but pass on a function that does fails beta reduction.

Fixes #21952

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

@@ -76,10 +76,10 @@ object BetaReduce:
     val bindingsBuf = new ListBuffer[DefTree]
     def recur(fn: Tree, argss: List[List[Tree]]): Option[Tree] = fn match
       case Block((ddef : DefDef) :: Nil, closure: Closure) if ddef.symbol == closure.meth.symbol =>
-        Some(reduceApplication(ddef, argss, bindingsBuf))
+        reduceApplication(ddef, argss, bindingsBuf)
       case Block((TypeDef(_, template: Template)) :: Nil, Typed(Apply(Select(New(_), _), _), _)) if template.constr.rhs.isEmpty =>
         template.body match
-          case (ddef: DefDef) :: Nil => Some(reduceApplication(ddef, argss, bindingsBuf))
+          case (ddef: DefDef) :: Nil => reduceApplication(ddef, argss, bindingsBuf)
           case _ => None
       case Block(stats, expr) if stats.forall(isPureBinding) =>
         recur(expr, argss).map(cpy.Block(fn)(stats, _))
@@ -106,12 +106,22 @@ object BetaReduce:
       case _ =>
         tree
 
-  /** Beta-reduces a call to `ddef` with arguments `args` and registers new bindings */
-  def reduceApplication(ddef: DefDef, argss: List[List[Tree]], bindings: ListBuffer[DefTree])(using Context): Tree =
+  /** Beta-reduces a call to `ddef` with arguments `args` and registers new bindings.
+   *  @return optionally, the expanded call, or none if the actual argument
+   *          lists do not match in shape the formal parameters
+   */
+  def reduceApplication(ddef: DefDef, argss: List[List[Tree]], bindings: ListBuffer[DefTree])
+      (using Context): Option[Tree] =
     val (targs, args) = argss.flatten.partition(_.isType)
     val tparams = ddef.leadingTypeParams
     val vparams = ddef.termParamss.flatten
 
+    def shapeMatch(paramss: List[ParamClause], argss: List[List[Tree]]): Boolean = (paramss, argss) match
+      case (params :: paramss1, args :: argss1) if params.length == args.length =>
+        shapeMatch(paramss1, argss1)
+      case (Nil, Nil) => true
+      case _ => false
+
     val targSyms =
       for (targ, tparam) <- targs.zip(tparams) yield
         targ.tpe.dealias match
@@ -143,19 +153,26 @@ object BetaReduce:
               bindings += binding.withSpan(arg.span)
             bindingSymbol
 
-    val expansion = TreeTypeMap(
-      oldOwners = ddef.symbol :: Nil,
-      newOwners = ctx.owner :: Nil,
-      substFrom = (tparams ::: vparams).map(_.symbol),
-      substTo = targSyms ::: argSyms
-    ).transform(ddef.rhs)
-
-    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 tpe: TypeRef if tree.isTerm && tpe.derivesFrom(defn.UnitClass) && isPureExpr(tree) =>
-          cpy.Literal(tree)(Constant(()))
-        case _ => super.transform(tree)
-    }.transform(expansion)
-
-    expansion1
+    if shapeMatch(ddef.paramss, argss) then
+      // We can't assume arguments always match. It's possible to construct a
+      // function with wrong apply method by hand which causes `shapeMatch` to fail.
+      // See neg/i21952.scala
+      val expansion = TreeTypeMap(
+        oldOwners = ddef.symbol :: Nil,
+        newOwners = ctx.owner :: Nil,
+        substFrom = (tparams ::: vparams).map(_.symbol),
+        substTo = targSyms ::: argSyms
+      ).transform(ddef.rhs)
+
+      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 tpe: TypeRef if tree.isTerm && tpe.derivesFrom(defn.UnitClass) && isPureExpr(tree) =>
+            cpy.Literal(tree)(Constant(()))
+          case _ => super.transform(tree)
+      }.transform(expansion)
+
+      Some(expansion1)
+    else None
+  end reduceApplication
+end BetaReduce

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

@@ -60,9 +60,11 @@ class InlinePatterns extends MiniPhase:
         template.body match
           case List(ddef @ DefDef(`name`, _, _, _)) =>
             val bindings = new ListBuffer[DefTree]()
-            val expansion1 = BetaReduce.reduceApplication(ddef, argss, bindings)
-            val bindings1 = bindings.result()
-            seq(bindings1, expansion1)
+            BetaReduce.reduceApplication(ddef, argss, bindings) match
+              case Some(expansion1) =>
+                val bindings1 = bindings.result()
+                seq(bindings1, expansion1)
+              case None => tree
           case _ => tree
       case _ => tree
 

tests/neg/i21952.scala

@@ -0,0 +1 @@
+val _ = (new Function[(Int, Int), Int] {def apply(a: Int, b: Int): Int = a * b})(2, 3) // error