Fix xorshift zero-seed lockup

include/exec/detail/xorshift.hpp

@@ -54,7 +54,7 @@ namespace experimental::execution
     friend auto operator==(xorshift const &, xorshift const &) -> bool = default;
 
     xorshift()
-      : m_seed(0xc1f651c67c62c6e0ull)
+      : m_seed(default_seed)
     {}
 
     explicit xorshift(std::random_device &rd)
@@ -63,12 +63,16 @@ namespace experimental::execution
     }
 
     explicit xorshift(std::uint64_t seed)
-      : m_seed(seed)
+      : m_seed(seed == 0 ? default_seed : seed)
     {}
 
     void seed(std::random_device &rd)
     {
       m_seed = std::uint64_t(rd()) << 31 | std::uint64_t(rd());
+      if (m_seed == 0)
+      {
+        m_seed = default_seed;
+      }
     }
 
     auto operator()() -> result_type
@@ -87,7 +91,8 @@ namespace experimental::execution
     }
 
    private:
-    std::uint64_t m_seed;
+    static constexpr std::uint64_t default_seed = 0xc1f651c67c62c6e0ull;
+    std::uint64_t                  m_seed;
   };
 
 }  // namespace experimental::execution

test/exec/CMakeLists.txt

@@ -45,6 +45,7 @@ set(exec_test_sources
     test_scheduler_concept.cpp
     test_sequence_senders.cpp
     test_static_thread_pool.cpp
+    test_xorshift.cpp
     test_just_from.cpp
     test_fork_join.cpp
     test_cpo_split.cpp

test/exec/test_xorshift.cpp

@@ -0,0 +1,41 @@
+/*
+ * Copyright (c) 2026 stdexec contributors
+ *
+ * Licensed under the Apache License Version 2.0 with LLVM Exceptions
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ *   https://llvm.org/LICENSE.txt
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include <exec/detail/xorshift.hpp>
+#include <test_common/catch2.hpp>
+
+#include <cstdint>
+
+TEST_CASE("xorshift does not get stuck with a zero seed", "[xorshift]")
+{
+  exec::xorshift zero_seed{std::uint64_t{0}};
+  exec::xorshift default_seed{};
+
+  bool produced_nonzero = false;
+  for (int i = 0; i < 8; ++i)
+  {
+    auto const sample = zero_seed();
+    CHECK(sample == default_seed());
+    produced_nonzero = produced_nonzero || sample != 0;
+  }
+  CHECK(produced_nonzero);
+}
+
+TEST_CASE("xorshift preserves nonzero seed behavior", "[xorshift]")
+{
+  exec::xorshift nonzero_seed{std::uint64_t{1}};
+  CHECK(nonzero_seed() == 0xd989bcacU);
+}