From 63146b4a60f89a4b3bdadcb241127a1671b0f949 Mon Sep 17 00:00:00 2001
From: odersky <odersky@gmail.com>
Date: Wed, 28 Feb 2024 22:20:47 +0100
Subject: [PATCH 1/2] Fix Function tree copier

It did not copy correctly instances of the FunctionWithMods subclass.

Fixes #19751
---
 compiler/src/dotty/tools/dotc/ast/Trees.scala |  9 ++++----
 compiler/src/dotty/tools/dotc/ast/untpd.scala |  6 ++++-
 tests/pos-custom-args/captures/i19751.scala   | 22 +++++++++++++++++++
 3 files changed, 32 insertions(+), 5 deletions(-)
 create mode 100644 tests/pos-custom-args/captures/i19751.scala

diff --git a/compiler/src/dotty/tools/dotc/ast/Trees.scala b/compiler/src/dotty/tools/dotc/ast/Trees.scala
index 4ec41b95a90b..05adedabf43b 100644
--- a/compiler/src/dotty/tools/dotc/ast/Trees.scala
+++ b/compiler/src/dotty/tools/dotc/ast/Trees.scala
@@ -1254,11 +1254,12 @@ object Trees {
         case _ => finalize(tree, untpd.Ident(name)(sourceFile(tree)))
       }
       def Select(tree: Tree)(qualifier: Tree, name: Name)(using Context): Select = tree match {
-        case tree: SelectWithSig =>
-          if ((qualifier eq tree.qualifier) && (name == tree.name)) tree
-          else finalize(tree, SelectWithSig(qualifier, name, tree.sig)(sourceFile(tree)))
         case tree: Select if (qualifier eq tree.qualifier) && (name == tree.name) => tree
-        case _ => finalize(tree, untpd.Select(qualifier, name)(sourceFile(tree)))
+        case _ =>
+          val tree1 = tree match
+            case tree: SelectWithSig => untpd.SelectWithSig(qualifier, name, tree.sig)(using sourceFile(tree))
+            case _ => untpd.Select(qualifier, name)(using sourceFile(tree))
+          finalize(tree, tree1)
       }
       /** Copy Ident or Select trees */
       def Ref(tree: RefTree)(name: Name)(using Context): RefTree = tree match {
diff --git a/compiler/src/dotty/tools/dotc/ast/untpd.scala b/compiler/src/dotty/tools/dotc/ast/untpd.scala
index aabfdd97d7bd..8afcfd9c6ac3 100644
--- a/compiler/src/dotty/tools/dotc/ast/untpd.scala
+++ b/compiler/src/dotty/tools/dotc/ast/untpd.scala
@@ -609,7 +609,11 @@ object untpd extends Trees.Instance[Untyped] with UntypedTreeInfo {
     }
     def Function(tree: Tree)(args: List[Tree], body: Tree)(using Context): Tree = tree match {
       case tree: Function if (args eq tree.args) && (body eq tree.body) => tree
-      case _ => finalize(tree, untpd.Function(args, body)(tree.source))
+      case _ =>
+        val tree1 = tree match
+          case tree: FunctionWithMods => untpd.FunctionWithMods(args, body, tree.mods, tree.erasedParams)(using tree.source)
+          case _ => untpd.Function(args, body)(using tree.source)
+        finalize(tree, tree1)
     }
     def PolyFunction(tree: Tree)(targs: List[Tree], body: Tree)(using Context): Tree = tree match {
       case tree: PolyFunction if (targs eq tree.targs) && (body eq tree.body) => tree
diff --git a/tests/pos-custom-args/captures/i19751.scala b/tests/pos-custom-args/captures/i19751.scala
new file mode 100644
index 000000000000..b6023cc0ff87
--- /dev/null
+++ b/tests/pos-custom-args/captures/i19751.scala
@@ -0,0 +1,22 @@
+import language.experimental.captureChecking
+import annotation.capability
+import caps.cap
+
+trait Ptr[A]
+@capability trait Scope:
+  def allocate(size: Int): Ptr[Unit]^{this}
+
+
+object Scope:
+  def confined[A](fn: Scope ?->{cap} A): A =
+    val scope = new Scope:
+      def allocate(size: Int) = new Ptr[Unit] {}
+    fn(using scope)
+
+def Test: Unit =
+  val s = Scope.confined:
+    val s2 = summon[Scope]
+    Scope.confined:
+      s2.allocate(5)
+    5
+

From cd68604247c56524631e841a93cfac89c2377cc6 Mon Sep 17 00:00:00 2001
From: odersky <odersky@gmail.com>
Date: Thu, 29 Feb 2024 11:13:19 +0100
Subject: [PATCH 2/2] Use fixed copying instead of writing code out

---
 compiler/src/dotty/tools/dotc/ast/Desugar.scala | 5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

diff --git a/compiler/src/dotty/tools/dotc/ast/Desugar.scala b/compiler/src/dotty/tools/dotc/ast/Desugar.scala
index 9591bc5a93f0..53287465614a 100644
--- a/compiler/src/dotty/tools/dotc/ast/Desugar.scala
+++ b/compiler/src/dotty/tools/dotc/ast/Desugar.scala
@@ -1978,10 +1978,7 @@ object desugar {
         val applyVParams = args.zipWithIndex.map {
           case (p, n) => makeSyntheticParameter(n + 1, p)
         }
-        tree match
-          case tree: FunctionWithMods =>
-            untpd.FunctionWithMods(applyVParams, result, tree.mods, tree.erasedParams)
-          case _ => untpd.Function(applyVParams, result)
+        cpy.Function(tree)(applyVParams, result).asInstanceOf[untpd.Function]
     }
   }