Fix by-name parameter in beta-reduction (#20096)

Fixes #20095

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,

tests/run/i20095.check

@@ -0,0 +1,2 @@
+foo
+foo

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)