From 879b926f90cb131e81703da107f33ed705c63519 Mon Sep 17 00:00:00 2001
From: shin4141 <128954611+shin4141@users.noreply.github.com>
Date: Sun, 23 Aug 2026 09:22:03 +0900
Subject: [PATCH] Fix BFE test for C++20 stateless lambdas

---
 doc/sphinx/docs/cpp/bfe.rst |  6 ++++--
 tests/bfe.cpp               | 20 +++++++++++++++++++-
 2 files changed, 23 insertions(+), 3 deletions(-)

diff --git a/doc/sphinx/docs/cpp/bfe.rst b/doc/sphinx/docs/cpp/bfe.rst
index 85c2a42a9..caacf4ec9 100644
--- a/doc/sphinx/docs/cpp/bfe.rst
+++ b/doc/sphinx/docs/cpp/bfe.rst
@@ -46,8 +46,10 @@ Batch fitness evaluator
    stored contiguously).
 
    Additionally, UDBFEs must also be destructible and default, copy and move constructible.
-   Note that pointers to plain C++ functions with an appropriate signature
-   are UDBFEs, but lambda functions are not (as they currently are not default-constructible).
+   Note that pointers to plain C++ functions with an appropriate signature are UDBFEs.
+   Stateless lambdas are also UDBFEs when their closure types are default-constructible
+   (e.g., when using C++20 or later), while capturing lambdas are not default-constructible
+   and thus are not UDBFEs.
 
    UDBFEs can also implement the following (optional) member functions:
 
diff --git a/tests/bfe.cpp b/tests/bfe.cpp
index cf32c2e34..4fba20d7d 100644
--- a/tests/bfe.cpp
+++ b/tests/bfe.cpp
@@ -455,12 +455,30 @@ BOOST_AUTO_TEST_CASE(s11n)
     BOOST_CHECK(bfe0.extract<udbfe_a>()->state = -42);
 }
 
+template <typename T>
+void test_lambda_udbfe(const T &fun)
+{
+    if constexpr (is_udbfe<T>::value) {
+        bfe bfe0{fun};
+        BOOST_CHECK(bfe0.is<T>());
+        bfe bfe1{bfe0};
+        BOOST_CHECK(bfe1.is<T>());
+        BOOST_CHECK(bfe1(problem{}, vector_double{.5}) == vector_double{1.});
+        BOOST_CHECK(bfe1(problem{null_problem{3}}, vector_double{.5}) == (vector_double{1., 1., 1.}));
+    }
+}
+
 BOOST_AUTO_TEST_CASE(lambda_std_function)
 {
     auto fun = [](const problem &p, const vector_double &dvs) {
         return vector_double(p.get_nf() * (dvs.size() / p.get_nx()), 1.);
     };
-    BOOST_CHECK(!is_udbfe<decltype(fun)>::value);
+    BOOST_CHECK(std::is_copy_constructible<decltype(fun)>::value);
+    BOOST_CHECK(std::is_move_constructible<decltype(fun)>::value);
+    BOOST_CHECK(std::is_destructible<decltype(fun)>::value);
+    BOOST_CHECK(has_bfe_call_operator<decltype(fun)>::value);
+    BOOST_CHECK_EQUAL(is_udbfe<decltype(fun)>::value, std::is_default_constructible<decltype(fun)>::value);
+    test_lambda_udbfe(fun);
 #if !defined(_MSC_VER)
     BOOST_CHECK(is_udbfe<decltype(+fun)>::value);
 #endif