Handle more corner cases in etaReduce

compiler/src/dotty/tools/dotc/core/Definitions.scala

@@ -1594,6 +1594,9 @@ class Definitions {
     yield
       nme.apply.specializedFunction(r, List(t1, t2)).asTermName
 
+  @tu lazy val FunctionSpecializedApplyNames: collection.Set[Name] =
+    Function0SpecializedApplyNames ++ Function1SpecializedApplyNames ++ Function2SpecializedApplyNames
+
   def functionArity(tp: Type)(using Context): Int = tp.dropDependentRefinement.dealias.argInfos.length - 1
 
   /** Return underlying context function type (i.e. instance of an ContextFunctionN class)

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

@@ -6,6 +6,8 @@ import MegaPhase.MiniPhase
 import core.*
 import Symbols.*, Contexts.*, Types.*, Decorators.*
 import StdNames.nme
+import SymUtils.*
+import NameKinds.AdaptedClosureName
 
 /** Rewrite `(x1, ... xN) => f(x1, ... xN)` for N >= 0 to `f`,
  *  provided `f` is a pure path of function type.
@@ -15,6 +17,11 @@ import StdNames.nme
  *  where a context function is expected, unless that value has the
  *  syntactic form of a context function literal.
  *
+ *  Also handle variants of eta-expansions where
+ *   - result f.apply(X_1,...,X_n) is subject to a synthetic cast, or
+ *   - the application uses a specialized apply method, or
+ *   - the closure is adapted (see Erasure#adaptClosure)
+ *
  *  Without this phase, when a contextual function is passed as an argument to a
  *  recursive function, that would have the unfortunate effect of a linear growth
  *  in transient thunks of identical type wrapped around each other, leading
@@ -27,20 +34,36 @@ class EtaReduce extends MiniPhase:
 
   override def description: String = EtaReduce.description
 
-  override def transformBlock(tree: Block)(using Context): Tree = tree match
-    case Block((meth : DefDef) :: Nil, closure: Closure)
-    if meth.symbol == closure.meth.symbol =>
-      meth.rhs match
-        case Apply(Select(fn, nme.apply), args)
-        if meth.paramss.head.corresponds(args)((param, arg) =>
+  override def transformBlock(tree: Block)(using Context): Tree =
+
+    def tryReduce(mdef: DefDef, rhs: Tree): Tree = rhs match
+      case Apply(Select(fn, name), args)
+      if (name == nme.apply || defn.FunctionSpecializedApplyNames.contains(name))
+          && mdef.paramss.head.corresponds(args)((param, arg) =>
               arg.isInstanceOf[Ident] && arg.symbol == param.symbol)
-            && isPurePath(fn)
-            && fn.tpe <:< tree.tpe
-            && defn.isFunctionClass(fn.tpe.widen.typeSymbol) =>
-          report.log(i"eta reducing $tree --> $fn")
-          fn
-        case _ => tree
-    case _ => tree
+          && isPurePath(fn)
+          && fn.tpe <:< tree.tpe
+          && defn.isFunctionClass(fn.tpe.widen.typeSymbol) =>
+        report.log(i"eta reducing $tree --> $fn")
+        fn
+      case TypeApply(Select(qual, _), _) if rhs.symbol.isTypeCast && rhs.span.isSynthetic =>
+        tryReduce(mdef, qual)
+      case _ =>
+        tree
+
+    tree match
+      case Block((meth: DefDef) :: Nil, expr) if meth.symbol.isAnonymousFunction =>
+        expr match
+          case closure: Closure if meth.symbol == closure.meth.symbol =>
+            tryReduce(meth, meth.rhs)
+          case Block((adapted: DefDef) :: Nil, closure: Closure)
+          if adapted.name.is(AdaptedClosureName) && adapted.symbol == closure.meth.symbol =>
+            tryReduce(meth, meth.rhs)
+          case _ =>
+            tree
+      case _ =>
+        tree
+  end transformBlock
 
 end EtaReduce
 

tests/run/i14623.scala

@@ -0,0 +1,15 @@
+object Thunk {
+  private[this] val impl =
+    ((x: Any) => x).asInstanceOf[(=> Any) => Function0[Any]]
+
+  def asFunction0[A](thunk: => A): Function0[A] = impl(thunk).asInstanceOf[Function0[A]]
+}
+
+@main def Test =
+  var i = 0
+  val f1 = { () => i += 1; "" }
+  assert(Thunk.asFunction0(f1()) eq f1)
+  val f2 = { () => i += 1; i }
+  assert(Thunk.asFunction0(f2()) eq f2)
+  val f3 = { () => i += 1 }
+  assert(Thunk.asFunction0(f3()) eq f3)