Implement LWG-3809 Is subtract_with_carry_engine<uint16_t> supposed to work?

stl/inc/random

@@ -677,9 +677,9 @@ public:
     using _Mybase     = _Circ_buf<_Ty, _Rx>;
     using _Seed_t     = typename _Swc_Traits::_Seed_t;
 
-    static constexpr size_t short_lag     = _Sx;
-    static constexpr size_t long_lag      = _Rx;
-    static constexpr _Seed_t default_seed = static_cast<_Seed_t>(19780503U);
+    static constexpr size_t short_lag            = _Sx;
+    static constexpr size_t long_lag             = _Rx;
+    static constexpr uint_least32_t default_seed = 19780503u;
 
     _Swc_base() {
         seed();
@@ -695,7 +695,7 @@ public:
     }
 
     void seed(_Seed_t _Value = default_seed, bool _Readcy = false) { // set initial values from specified seed value
-        linear_congruential<_Seed_t, 40014U, 0U, 2147483563U> _Lc{_Value == 0U ? default_seed : _Value};
+        linear_congruential_engine<uint_least32_t, 40014U, 0U, 2147483563U> _Lc{_Value == 0U ? default_seed : _Value};
         _Reset(_Lc, _Readcy);
     }
 
@@ -805,7 +805,7 @@ struct _Swc_traits { // traits for subtract_with_carry generator
 
     static constexpr _Cy_t _Cy   = 1;
     static constexpr _Mod_t _Mod = _Mx;
-    static constexpr _Ty _Max    = _Mx - 1;
+    static constexpr _Ty _Max    = static_cast<_Ty>(_Mx - 1);
 
     static int _Get_wc() noexcept { // compute number of 32-bit words per element
         int _Kx;
@@ -901,15 +901,16 @@ public:
 };
 
 _EXPORT_STD template <class _Ty, size_t _Wx, size_t _Sx, size_t _Rx>
-class subtract_with_carry_engine : public subtract_with_carry<_Ty, (_Ty{1} << (_Wx - 1)) << 1, _Sx, _Rx> {
+class subtract_with_carry_engine
+    : public subtract_with_carry<_Ty, static_cast<_Ty>((_Ty{1} << (_Wx - 1)) << 1), _Sx, _Rx> {
     // subtract_with_carry generator
 public:
     _RNG_REQUIRE_UINTTYPE(subtract_with_carry_engine, _Ty);
 
     static_assert(0U < _Sx && _Sx < _Rx && 0 < _Wx && _Wx <= numeric_limits<_Ty>::digits,
         "invalid template argument for subtract_with_carry_engine");
 
-    static constexpr _Ty _Mx          = (_Ty{1} << (_Wx - 1)) << 1;
+    static constexpr _Ty _Mx          = static_cast<_Ty>((_Ty{1} << (_Wx - 1)) << 1);
     static constexpr size_t word_size = _Wx;
     static constexpr size_t short_lag = _Sx;
     static constexpr size_t long_lag  = _Rx;
@@ -920,7 +921,7 @@ public:
 
     using _Mybase::default_seed;
 
-    subtract_with_carry_engine() : _Mybase(default_seed) {}
+    subtract_with_carry_engine() : _Mybase(0u) {}
 
     explicit subtract_with_carry_engine(_Ty _Xx0) : _Mybase(_Xx0) {}
 
@@ -929,7 +930,7 @@ public:
         seed(_Seq);
     }
 
-    void seed(_Ty _Value = default_seed) { // set initial values from specified seed value
+    void seed(_Ty _Value = 0u) { // set initial values from specified seed value
         _Mybase::seed(_Value);
     }
 

tests/std/tests/Dev11_0577418_random_seed_0/test.cpp

@@ -2,7 +2,11 @@
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
 #include <cassert>
+#include <cstdint>
 #include <random>
+#include <type_traits>
+
+#define STATIC_ASSERT(...) static_assert(__VA_ARGS__, #__VA_ARGS__)
 
 using namespace std;
 
@@ -13,20 +17,31 @@ typename Engine::result_type run_10k(Engine engine) {
 }
 
 int main() {
-    assert(run_10k(minstd_rand0()) == 1043618065UL); // N3485 26.5.5 [rand.predef]/1
-    assert(run_10k(mt19937()) == 4123659995UL); // N3485 26.5.5 [rand.predef]/3
-    assert(run_10k(ranlux24_base()) == 7937952UL); // N3485 26.5.5 [rand.predef]/5
+    assert(run_10k(minstd_rand0()) == 1043618065UL); // N4964 [rand.predef]/1
+    assert(run_10k(mt19937()) == 4123659995UL); // N4964 [rand.predef]/3
+    assert(run_10k(ranlux24_base()) == 7937952UL); // N4964 [rand.predef]/5
 
     // Also test VSO-214595 "subtract_with_carry_engine::seed should accept result_type"
     subtract_with_carry_engine<unsigned long long, 64, 10, 24> ull_swc;
     ull_swc.seed(0x12341234'00000000ULL);
-    assert(run_10k(ull_swc) == 0x01316AEA'3646F686ULL); // libstdc++ and libc++ agree (boost 1.60.0 disagrees)
+    assert(run_10k(ull_swc) == 0x1DD6C263'C41EEED0ULL); // value changed by LWG-3809
 
-    assert(minstd_rand0(0) == minstd_rand0()); // N3485 26.5.3.1 [rand.eng.lcong]/5
-    assert(mt19937(0) != mt19937()); // N3485 26.5.3.2 [rand.eng.mers]/6
-    assert(ranlux24_base(0) == ranlux24_base()); // N3485 26.5.3.3 [rand.eng.sub]/7
+    assert(minstd_rand0(0) == minstd_rand0()); // N4964 [rand.eng.lcong]/5
+    assert(mt19937(0) != mt19937()); // N4964 [rand.eng.mers]/6
+    assert(ranlux24_base(0) == ranlux24_base()); // N4964 [rand.eng.sub]/7
 
     assert(run_10k(minstd_rand0(0)) == 1043618065UL); // QED
     assert(run_10k(mt19937(0)) == 1543171712UL); // Boost 1.52.0 agrees
     assert(run_10k(ranlux24_base(0)) == 7937952UL); // QED
+
+    // Also test LWG-3809 Is std::subtract_with_carry_engine<uint16_t> supposed to work?
+    STATIC_ASSERT(is_same_v<decltype(subtract_with_carry_engine<unsigned short, 16, 10, 24>::default_seed),
+        const uint_least32_t>);
+    STATIC_ASSERT(
+        is_same_v<decltype(subtract_with_carry_engine<unsigned int, 32, 10, 24>::default_seed), const uint_least32_t>);
+    STATIC_ASSERT(
+        is_same_v<decltype(subtract_with_carry_engine<unsigned long, 32, 10, 24>::default_seed), const uint_least32_t>);
+    STATIC_ASSERT(is_same_v<decltype(subtract_with_carry_engine<unsigned long long, 64, 10, 24>::default_seed),
+        const uint_least32_t>);
+    assert(run_10k(subtract_with_carry_engine<unsigned long long, 64, 10, 24>{}) == 6793538734622947770ULL);
 }