From e8a85e6ebae39a10c2dd9e257f045725925f4d47 Mon Sep 17 00:00:00 2001
From: Nicolas Stucki <nicolas.stucki@gmail.com>
Date: Wed, 25 Oct 2023 13:36:24 +0200
Subject: [PATCH] Properly handle by-name function types in repl

Fixes #18756
---
 .../dotty/tools/dotc/transform/ElimByName.scala   | 15 +++++++++++----
 .../dotc/transform/SpecializeFunctions.scala      |  5 ++++-
 compiler/test-resources/repl/i18756               |  5 +++++
 3 files changed, 20 insertions(+), 5 deletions(-)
 create mode 100644 compiler/test-resources/repl/i18756

diff --git a/compiler/src/dotty/tools/dotc/transform/ElimByName.scala b/compiler/src/dotty/tools/dotc/transform/ElimByName.scala
index 151e841f0e48..225541ba813a 100644
--- a/compiler/src/dotty/tools/dotc/transform/ElimByName.scala
+++ b/compiler/src/dotty/tools/dotc/transform/ElimByName.scala
@@ -131,12 +131,19 @@ class ElimByName extends MiniPhase, InfoTransformer:
 
   override def transformApply(tree: Apply)(using Context): Tree =
     trace(s"transforming ${tree.show} at phase ${ctx.phase}", show = true) {
-
+      def stripTyped(t: Tree): Tree = t match
+        case Typed(expr, _) => stripTyped(expr)
+        case _ => t
       def transformArg(arg: Tree, formal: Type): Tree = formal match
         case defn.ByNameFunction(formalResult) =>
-          def stripTyped(t: Tree): Tree = t match
-            case Typed(expr, _) => stripTyped(expr)
-            case _ => t
+          stripTyped(arg) match
+            case Apply(Select(qual, nme.apply), Nil)
+            if isByNameRef(qual) && (isPureExpr(qual) || qual.symbol.isAllOf(InlineParam)) =>
+              qual
+            case _ =>
+              if isByNameRef(arg) || arg.symbol.name.is(SuperArgName) then arg
+              else byNameClosure(arg, formalResult)
+        case ExprType(formalResult) =>
           stripTyped(arg) match
             case Apply(Select(qual, nme.apply), Nil)
             if isByNameRef(qual) && (isPureExpr(qual) || qual.symbol.isAllOf(InlineParam)) =>
diff --git a/compiler/src/dotty/tools/dotc/transform/SpecializeFunctions.scala b/compiler/src/dotty/tools/dotc/transform/SpecializeFunctions.scala
index c50eaddd3213..4eef58b77a5e 100644
--- a/compiler/src/dotty/tools/dotc/transform/SpecializeFunctions.scala
+++ b/compiler/src/dotty/tools/dotc/transform/SpecializeFunctions.scala
@@ -71,7 +71,10 @@ class SpecializeFunctions extends MiniPhase {
   override def transformApply(tree: Apply)(using Context) =
     tree match {
       case Apply(fun: NameTree, args) if fun.name == nme.apply && args.size <= 3 && fun.symbol.maybeOwner.isType =>
-        val argTypes = fun.tpe.widen.firstParamTypes.map(_.widenSingleton.dealias)
+        val argTypes = fun.tpe.widen.firstParamTypes.map(_.widenSingleton.dealias).map {
+          case ExprType(resType) => defn.FunctionOf(Nil, resType, isContextual = true)
+          case arg => arg
+        }
         val retType  = tree.tpe.widenSingleton.dealias
         val isSpecializable =
           defn.isSpecializableFunction(
diff --git a/compiler/test-resources/repl/i18756 b/compiler/test-resources/repl/i18756
new file mode 100644
index 000000000000..722c665999b6
--- /dev/null
+++ b/compiler/test-resources/repl/i18756
@@ -0,0 +1,5 @@
+scala> def f: ( => Int) => Int = i => i ; f(1)
+def f: (=> Int) => Int
+val res0: Int = 1
+scala> f(1)
+val res1: Int = 1