{"id":2231568,"url":"http://patchwork.ozlabs.org/api/1.1/patches/2231568/?format=json","web_url":"http://patchwork.ozlabs.org/project/gcc/patch/20260430210040.2360840-1-jwakely@redhat.com/","project":{"id":17,"url":"http://patchwork.ozlabs.org/api/1.1/projects/17/?format=json","name":"GNU Compiler Collection","link_name":"gcc","list_id":"gcc-patches.gcc.gnu.org","list_email":"gcc-patches@gcc.gnu.org","web_url":null,"scm_url":null,"webscm_url":null},"msgid":"<20260430210040.2360840-1-jwakely@redhat.com>","date":"2026-04-30T21:00:04","name":"libstdc++: Do not assume URBG::result_type exists [PR121919]","commit_ref":null,"pull_url":null,"state":"new","archived":false,"hash":"e7e8d655a0e0d088ea7cbcfe3810cf8b60871dbc","submitter":{"id":48004,"url":"http://patchwork.ozlabs.org/api/1.1/people/48004/?format=json","name":"Jonathan Wakely","email":"jwakely@redhat.com"},"delegate":null,"mbox":"http://patchwork.ozlabs.org/project/gcc/patch/20260430210040.2360840-1-jwakely@redhat.com/mbox/","series":[{"id":502385,"url":"http://patchwork.ozlabs.org/api/1.1/series/502385/?format=json","web_url":"http://patchwork.ozlabs.org/project/gcc/list/?series=502385","date":"2026-04-30T21:00:04","name":"libstdc++: Do not assume URBG::result_type exists [PR121919]","version":1,"mbox":"http://patchwork.ozlabs.org/series/502385/mbox/"}],"comments":"http://patchwork.ozlabs.org/api/patches/2231568/comments/","check":"pending","checks":"http://patchwork.ozlabs.org/api/patches/2231568/checks/","tags":{},"headers":{"Return-Path":"","X-Original-To":["incoming@patchwork.ozlabs.org","gcc-patches@gcc.gnu.org"],"Delivered-To":["patchwork-incoming@legolas.ozlabs.org","gcc-patches@gcc.gnu.org"],"Authentication-Results":["legolas.ozlabs.org;\n\tdkim=pass (1024-bit key;\n unprotected) header.d=redhat.com header.i=@redhat.com header.a=rsa-sha256\n header.s=mimecast20190719 header.b=hfbEb5m+;\n\tdkim-atps=neutral","legolas.ozlabs.org;\n spf=pass (sender SPF authorized) smtp.mailfrom=gcc.gnu.org\n (client-ip=38.145.34.32; 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That isn't needed\nto satisfy the concept. Change the algorithms to use decltype(__g())\ninstead of using result_type.\n\nThis isn't sufficient to fix the bug though, because those algorithms\nuse std::uniform_int_distribution and that class template's operator()\noverloads depend on the more restrictive uniform random bit generator\nrequirements, which do include the presence of a nested result_type\nmember.\n\nWe need to change std::uniform_int_distribution to also use decltype\ninstead of the nested result_type, even though the standard says that\nstd::uniform_int_distribution is allowed to assume that result_type\nexists.\n\nThere's yet another problem, which is that a type that returns random\nbool values can model the concept, but doesn't meet the named\nrequirements and can't be used with std::uniform_int_distribution. That\nisn't addressed by this change.\n\nlibstdc++-v3/ChangeLog:\n\n\tPR libstdc++/121919\n\t* include/bits/ranges_algo.h (__sample_fn, __shuffle_fn): Use\n\tdecltype(__g()) instead of remove_reference_t<_G>::result_type.\n\t* include/bits/uniform_int_dist.h\n\t(uniform_int_distribution::operator()): Use decltype(__urng())\n\tinstead of _UniformRandomBitGenerator::result_type\n\t(uniform_int_distribution::__generate_impl): Likewise.\n\t* testsuite/25_algorithms/sample/121919.cc: New test.\n\t* testsuite/25_algorithms/shuffle/121919.cc: New test.\n---\n\nTested x86_64-linux.\n\nI'm opening an LWG issue about this topic, because this is a silly\nsituation we've created.\n\n libstdc++-v3/include/bits/ranges_algo.h | 7 ++---\n libstdc++-v3/include/bits/uniform_int_dist.h | 4 +--\n .../testsuite/25_algorithms/sample/121919.cc | 28 +++++++++++++++++++\n .../testsuite/25_algorithms/shuffle/121919.cc | 25 +++++++++++++++++\n 4 files changed, 57 insertions(+), 7 deletions(-)\n create mode 100644 libstdc++-v3/testsuite/25_algorithms/sample/121919.cc\n create mode 100644 libstdc++-v3/testsuite/25_algorithms/shuffle/121919.cc","diff":"diff --git a/libstdc++-v3/include/bits/ranges_algo.h b/libstdc++-v3/include/bits/ranges_algo.h\nindex 0d1928d52a2f..4330d3e70b84 100644\n--- a/libstdc++-v3/include/bits/ranges_algo.h\n+++ b/libstdc++-v3/include/bits/ranges_algo.h\n@@ -1850,8 +1850,7 @@ namespace ranges\n \t using __distrib_type = uniform_int_distribution<_Size>;\n \t using __param_type = typename __distrib_type::param_type;\n \t using _USize = __detail::__make_unsigned_like_t<_Size>;\n-\t using __uc_type\n-\t = common_type_t::result_type, _USize>;\n+\t using __uc_type = common_type_t;\n \n \t if (__first == __last)\n \t return __out;\n@@ -1964,9 +1963,7 @@ namespace ranges\n \tusing __ud_type = __detail::__make_unsigned_like_t<_DistanceType>;\n \tusing __distr_type = std::uniform_int_distribution<__ud_type>;\n \tusing __p_type = typename __distr_type::param_type;\n-\n-\tusing __uc_type\n-\t = common_type_t::result_type, __ud_type>;\n+\tusing __uc_type = common_type_t;\n \n \tif constexpr (sized_sentinel_for<_Sent, _Iter>)\n \t {\ndiff --git a/libstdc++-v3/include/bits/uniform_int_dist.h b/libstdc++-v3/include/bits/uniform_int_dist.h\nindex dcf763ae7649..9c5514c7c138 100644\n--- a/libstdc++-v3/include/bits/uniform_int_dist.h\n+++ b/libstdc++-v3/include/bits/uniform_int_dist.h\n@@ -288,7 +288,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION\n operator()(_UniformRandomBitGenerator& __urng,\n \t\t const param_type& __param)\n {\n-\ttypedef typename _UniformRandomBitGenerator::result_type _Gresult_type;\n+\ttypedef decltype(__urng()) _Gresult_type;\n \ttypedef typename make_unsigned::type __utype;\n \ttypedef typename common_type<_Gresult_type, __utype>::type __uctype;\n \n@@ -386,7 +386,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION\n \t\t const param_type& __param)\n {\n \t__glibcxx_function_requires(_ForwardIteratorConcept<_ForwardIterator>)\n-\ttypedef typename _UniformRandomBitGenerator::result_type _Gresult_type;\n+\ttypedef decltype(__urng()) _Gresult_type;\n \ttypedef typename make_unsigned::type __utype;\n \ttypedef typename common_type<_Gresult_type, __utype>::type __uctype;\n \ndiff --git a/libstdc++-v3/testsuite/25_algorithms/sample/121919.cc b/libstdc++-v3/testsuite/25_algorithms/sample/121919.cc\nnew file mode 100644\nindex 000000000000..9fdfbf560c71\n--- /dev/null\n+++ b/libstdc++-v3/testsuite/25_algorithms/sample/121919.cc\n@@ -0,0 +1,28 @@\n+// { dg-do compile { target c++20 } }\n+\n+// Bug 121919 ranges::sample assumes a uniform_random_bit_generator\n+// provides result_type\n+\n+#include \n+#include \n+\n+struct G\n+{\n+ constexpr static unsigned min() { return 0; }\n+ constexpr static unsigned max() { return 10; }\n+ unsigned operator()() const;\n+};\n+\n+static_assert(std::uniform_random_bit_generator);\n+\n+void\n+test_pr121919()\n+{\n+ int i2[2]{ 1, 2 };\n+ __gnu_test::test_random_access_range from(i2);\n+ int i1[1];\n+ __gnu_test::test_random_access_range to(i1);\n+ std::ranges::sample(from, std::ranges::begin(to), 1, G{});\n+ std::ranges::sample(std::ranges::begin(from), std::ranges::end(to),\n+\t\t std::ranges::begin(to), 1, G{});\n+}\ndiff --git a/libstdc++-v3/testsuite/25_algorithms/shuffle/121919.cc b/libstdc++-v3/testsuite/25_algorithms/shuffle/121919.cc\nnew file mode 100644\nindex 000000000000..9425394c50ee\n--- /dev/null\n+++ b/libstdc++-v3/testsuite/25_algorithms/shuffle/121919.cc\n@@ -0,0 +1,25 @@\n+// { dg-do compile { target c++20 } }\n+\n+// Bug 121919 ranges::shuffle assumes a uniform_random_bit_generator\n+// provides result_type\n+\n+#include \n+#include \n+\n+struct G\n+{\n+ constexpr static unsigned min() { return 0; }\n+ constexpr static unsigned max() { return 10; }\n+ unsigned operator()() const;\n+};\n+\n+static_assert(std::uniform_random_bit_generator);\n+\n+void\n+test_pr121919()\n+{\n+ int arr[2]{ 1, 2 };\n+ __gnu_test::test_random_access_range r(arr);\n+ std::ranges::shuffle(r, G{});\n+ std::ranges::shuffle(std::ranges::begin(r), std::ranges::end(r), G{});\n+}\n","prefixes":[]}