[llvm-branch-commits] [libcxx] release/21.x: [libc++] Fix std::variant evaluating template arguments too eagerly (#151028) (PR #153064)

[via llvm-branch-commits]

https://github.com/llvmbot created https://github.com/llvm/llvm-project/pull/153064

Backport f5f582451e913581ef214b8b771a13d3912394d5

Requested by: @ldionne

>From bdb0752852d61a40951cb6152208fd2e2d313b25 Mon Sep 17 00:00:00 2001
From: Nikolas Klauser <nikolasklauser at berlin.de>
Date: Mon, 11 Aug 2025 20:15:59 +0200
Subject: [PATCH] [libc++] Fix std::variant evaluating template arguments too
 eagerly (#151028)

This has been reported in https://github.com/llvm/llvm-project/pull/116709#issuecomment-3105095648.

Fixes #151328

(cherry picked from commit f5f582451e913581ef214b8b771a13d3912394d5)
---
 libcxx/include/__type_traits/invoke.h            | 10 +++++-----
 .../variant.variant/variant.ctor/T.pass.cpp      | 16 ++++++++++++++++
 2 files changed, 21 insertions(+), 5 deletions(-)

diff --git a/libcxx/include/__type_traits/invoke.h b/libcxx/include/__type_traits/invoke.h
index 5ff2efbe5faaf..3f5626c014432 100644
--- a/libcxx/include/__type_traits/invoke.h
+++ b/libcxx/include/__type_traits/invoke.h
@@ -67,20 +67,20 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 
 #if __has_builtin(__builtin_invoke)
 
-template <class... _Args>
-using __invoke_result_t _LIBCPP_NODEBUG = decltype(__builtin_invoke(std::declval<_Args>()...));
-
 template <class, class... _Args>
 struct __invoke_result_impl {};
 
 template <class... _Args>
-struct __invoke_result_impl<__void_t<__invoke_result_t<_Args...> >, _Args...> {
-  using type _LIBCPP_NODEBUG = __invoke_result_t<_Args...>;
+struct __invoke_result_impl<__void_t<decltype(__builtin_invoke(std::declval<_Args>()...))>, _Args...> {
+  using type _LIBCPP_NODEBUG = decltype(__builtin_invoke(std::declval<_Args>()...));
 };
 
 template <class... _Args>
 using __invoke_result _LIBCPP_NODEBUG = __invoke_result_impl<void, _Args...>;
 
+template <class... _Args>
+using __invoke_result_t _LIBCPP_NODEBUG = typename __invoke_result<_Args...>::type;
+
 template <class... _Args>
 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR __invoke_result_t<_Args...> __invoke(_Args&&... __args)
     _NOEXCEPT_(noexcept(__builtin_invoke(std::forward<_Args>(__args)...))) {
diff --git a/libcxx/test/std/utilities/variant/variant.variant/variant.ctor/T.pass.cpp b/libcxx/test/std/utilities/variant/variant.variant/variant.ctor/T.pass.cpp
index 142da1d820d9a..6111138726dbc 100644
--- a/libcxx/test/std/utilities/variant/variant.variant/variant.ctor/T.pass.cpp
+++ b/libcxx/test/std/utilities/variant/variant.variant/variant.ctor/T.pass.cpp
@@ -173,6 +173,11 @@ void test_vector_bool() {
   assert(std::get<0>(v) == true);
 }
 
+struct ConvertibleFromAny {
+  template <class V>
+  ConvertibleFromAny(V) {}
+};
+
 int main(int, char**) {
   test_T_ctor_basic();
   test_T_ctor_noexcept();
@@ -180,5 +185,16 @@ int main(int, char**) {
   test_no_narrowing_check_for_class_types();
   test_construction_with_repeated_types();
   test_vector_bool();
+
+  { // Check that the constraints are evaluated lazily (see https://github.com/llvm/llvm-project/issues/151328)
+    struct Matcher {
+      Matcher() {}
+      Matcher(std::variant<ConvertibleFromAny>) {}
+    };
+
+    Matcher vec;
+    [[maybe_unused]] Matcher m = std::move(vec);
+  }
+
   return 0;
 }