Runtime: Lift prohibition on packs in swift_getTypeByMangledName() overload used by mirrors

Fixes rdar://problem/112866068.

Author: slavapestov

stdlib/public/runtime/ReflectionMirror.cpp

@@ -379,8 +379,7 @@ getFieldAt(const Metadata *base, unsigned index) {
   auto result = swift_getTypeByMangledName(
       MetadataState::Complete, typeName, substitutions.getGenericArgs(),
       [&substitutions](unsigned depth, unsigned index) {
-        // FIXME: Variadic generics
-        return substitutions.getMetadata(depth, index).getMetadata();
+        return substitutions.getMetadata(depth, index).Ptr;
       },
       [&substitutions](const Metadata *type, unsigned index) {
         return substitutions.getWitnessTable(type, index);

test/stdlib/MirrorWithPacks.swift

@@ -0,0 +1,51 @@
+//===--- MirrorWithPacks.swift -----------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2023 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+// RUN: %empty-directory(%t)
+// RUN: cp %s %t/main.swift
+// RUN: %target-build-swift %t/main.swift -o %t/Mirror -Xfrontend -disable-availability-checking
+// RUN: %target-codesign %t/Mirror
+// RUN: %target-run %t/Mirror
+
+// REQUIRES: executable_test
+// REQUIRES: shell
+// REQUIRES: reflection
+
+// rdar://96439408
+// UNSUPPORTED: use_os_stdlib
+
+import StdlibUnittest
+
+var mirrors = TestSuite("MirrorWithPacks")
+
+struct Tuple<each T> {
+  var elements: (repeat each T)
+  init(_ elements: repeat each T) {
+    self.elements = (repeat each elements)
+  }
+}
+
+mirrors.test("Packs") {
+  let value = Tuple("hi", 3, false)
+  var output = ""
+  dump(value, to: &output)
+
+  let expected =
+    "▿ Mirror.Tuple<Pack{Swift.String, Swift.Int, Swift.Bool}>\n" +
+    "  ▿ elements: (3 elements)\n" +
+    "    - .0: \"hi\"\n" +
+    "    - .1: 3\n" +
+    "    - .2: false\n"
+
+  expectEqual(expected, output)
+}
+
+runAllTests()