From 0b512d681275f9ebeeed99275579664f8a9deea4 Mon Sep 17 00:00:00 2001
From: Nicolas Stucki <nicolas.stucki@gmail.com>
Date: Fri, 5 Apr 2024 09:34:59 +0200
Subject: [PATCH] Fix by-name parameter in beta-reduction

Fixes #20095
---
 .../dotty/tools/dotc/transform/BetaReduce.scala | 17 +++++++++++------
 tests/run/i20095.check                          |  2 ++
 tests/run/i20095.scala                          |  9 +++++++++
 3 files changed, 22 insertions(+), 6 deletions(-)
 create mode 100644 tests/run/i20095.check
 create mode 100644 tests/run/i20095.scala

diff --git a/compiler/src/dotty/tools/dotc/transform/BetaReduce.scala b/compiler/src/dotty/tools/dotc/transform/BetaReduce.scala
index 720ed45bbe6c..dd20ff9557ca 100644
--- a/compiler/src/dotty/tools/dotc/transform/BetaReduce.scala
+++ b/compiler/src/dotty/tools/dotc/transform/BetaReduce.scala
@@ -128,15 +128,20 @@ object BetaReduce:
           case ref @ TermRef(NoPrefix, _) if isPurePath(arg) =>
             ref.symbol
           case _ =>
-            val flags = Synthetic | (param.symbol.flags & Erased)
-            val tpe =
+            val isByNameArg = param.tpt.tpe.isInstanceOf[ExprType]
+            val flags =
+              if isByNameArg then Synthetic | Method | (param.symbol.flags & Erased)
+              else Synthetic | (param.symbol.flags & Erased)
+            val tpe0 =
               if arg.tpe.isBottomType then param.tpe.widenTermRefExpr
               else 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)
-            if !((tpe.isInstanceOf[ConstantType] || tpe.derivesFrom(defn.UnitClass)) && isPureExpr(arg)) then
-              bindings += binding
-            binding.symbol
+            val tpe = if isByNameArg then ExprType(tpe0) else tpe0
+            val bindingSymbol = newSymbol(ctx.owner, param.name, flags, tpe, coord = arg.span)
+            val binding = if isByNameArg then DefDef(bindingSymbol, arg) else ValDef(bindingSymbol, arg)
+            if isByNameArg || !((tpe.isInstanceOf[ConstantType] || tpe.derivesFrom(defn.UnitClass)) && isPureExpr(arg)) then
+              bindings += binding.withSpan(arg.span)
+            bindingSymbol
 
     val expansion = TreeTypeMap(
       oldOwners = ddef.symbol :: Nil,
diff --git a/tests/run/i20095.check b/tests/run/i20095.check
new file mode 100644
index 000000000000..0d55bed3a35c
--- /dev/null
+++ b/tests/run/i20095.check
@@ -0,0 +1,2 @@
+foo
+foo
diff --git a/tests/run/i20095.scala b/tests/run/i20095.scala
new file mode 100644
index 000000000000..e5761f49b539
--- /dev/null
+++ b/tests/run/i20095.scala
@@ -0,0 +1,9 @@
+inline def twice(inline thunk: =>Unit): Unit =
+  thunk
+  thunk
+
+inline def pipe(inline t: =>Unit, inline f: (=>Unit) => Unit): Unit = f(t)
+
+@main def Test =
+  pipe((), twice)
+  pipe(println("foo"), twice)