libstdc++: Make 16-bit std::subtract_with_carry_engine work [PR107466]

This implements the proposed resolution of LWG 3809, so that
std::subtract_with_carry_engine can be used with a 16-bit result_type.
Currently this produces a narrowing error when instantiating the
std::linear_congruential_engine to create the initial state. It also
truncates the default_seed constant when passing it as a result_type
argument.

Change the type of the constant to uint_least32_t and pass 0u when the
default_seed should be used.

libstdc++-v3/ChangeLog:

	PR libstdc++/107466
	* include/bits/random.h (subtract_with_carry_engine): Use 32-bit
	type for default seed. Use 0u as default argument for
	subtract_with_carry_engine(result_type) constructor and
	seed(result_type) member function.
	* include/bits/random.tcc (subtract_with_carry_engine): Use
	32-bit type for default seed and engine used for initial state.
	* testsuite/26_numerics/random/subtract_with_carry_engine/cons/lwg3809.cc:
	New test.

3 files changed, 31 insertions, 5 deletions

diff --git a/libstdc++-v3/include/bits/random.h b/libstdc++-v3/include/bits/random.h
index 3b4e7d4..523ef2d 100644
--- a/libstdc++-v3/include/bits/random.h
+++ b/libstdc++-v3/include/bits/random.h
@@ -721,9 +721,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       static constexpr size_t      word_size    = __w;
       static constexpr size_t      short_lag    = __s;
       static constexpr size_t      long_lag     = __r;
-      static constexpr result_type default_seed = 19780503u;
+      static constexpr uint_least32_t default_seed = 19780503u;
 
-      subtract_with_carry_engine() : subtract_with_carry_engine(default_seed)
+      subtract_with_carry_engine() : subtract_with_carry_engine(0u)
       { }
 
       /**
@@ -758,7 +758,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
        * set carry to 1, otherwise sets carry to 0.
        */
       void
-      seed(result_type __sd = default_seed);
+      seed(result_type __sd = 0u);
 
       /**
        * @brief Seeds the initial state @f$x_0@f$ of the
diff --git a/libstdc++-v3/include/bits/random.tcc b/libstdc++-v3/include/bits/random.tcc
index cb1d367..7ec2b3f 100644
--- a/libstdc++-v3/include/bits/random.tcc
+++ b/libstdc++-v3/include/bits/random.tcc
@@ -532,7 +532,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     subtract_with_carry_engine<_UIntType, __w, __s, __r>::long_lag;
 
   template<typename _UIntType, size_t __w, size_t __s, size_t __r>
-    constexpr _UIntType
+    constexpr uint_least32_t
     subtract_with_carry_engine<_UIntType, __w, __s, __r>::default_seed;
 #endif
 
@@ -541,7 +541,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     subtract_with_carry_engine<_UIntType, __w, __s, __r>::
     seed(result_type __value)
     {
-      std::linear_congruential_engine<result_type, 40014u, 0u, 2147483563u>
+      std::linear_congruential_engine<uint_least32_t, 40014u, 0u, 2147483563u>
 	__lcg(__value == 0u ? default_seed : __value);
 
       const size_t __n = (__w + 31) / 32;
diff --git a/libstdc++-v3/testsuite/26_numerics/random/subtract_with_carry_engine/cons/lwg3809.cc b/libstdc++-v3/testsuite/26_numerics/random/subtract_with_carry_engine/cons/lwg3809.cc
new file mode 100644
index 0000000..21f246b
--- /dev/null
+++ b/libstdc++-v3/testsuite/26_numerics/random/subtract_with_carry_engine/cons/lwg3809.cc
@@ -0,0 +1,26 @@
+// { dg-do run { target c++11 } }
+#include <random>
+#include <testsuite_hooks.h>
+
+// LWG 3809. Is std::subtract_with_carry_engine<uint16_t> supposed to work?
+// PR 107466 - invalid -Wnarrowing error with std::subtract_with_carry_engine
+
+int main()
+{
+  // It should be possible to construct this engine with a 16-bit result_type:
+  std::subtract_with_carry_engine<uint16_t, 12, 5, 12> s16;
+  std::subtract_with_carry_engine<uint32_t, 12, 5, 12> s32;
+  // It should have been seeded with the same sequence as the 32-bit version
+  // and produce random numbers in the same range, [0, 1<<12).
+  for (int i = 0; i < 10; ++i)
+    VERIFY( s16() == s32() );
+  // The default seed should be usable without truncation to uint16_t:
+  s16.seed();
+  s32.seed();
+  for (int i = 0; i < 10; ++i)
+    VERIFY( s16() == s32() );
+  s16.seed(101);
+  s32.seed(101);
+  for (int i = 0; i < 10; ++i)
+    VERIFY( s16() == s32() );
+}