[Bug libstdc++/107466] [12 Regression] invalid -Wnarrowing error with std::subtract_with_carry_engine

[redi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107466

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |REOPENED
         Resolution|FIXED                       |---

--- Comment #9 from Jonathan Wakely <redi at gcc dot gnu.org> ---
I'm reopening this because the fix changes the behaviour of some code:

https://cplusplus.github.io/LWG/issue4014

As noted there, std::ranlux48_base(UINT_MAX+1LL)() changed behaviour.

Originally that yielded 22575453646312 but after the commits above it yields
23223501020940

This is what I proposed for 4014 and restores the original behaviour:

--- a/libstdc++-v3/include/bits/random.tcc
+++ b/libstdc++-v3/include/bits/random.tcc
@@ -541,8 +541,16 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     subtract_with_carry_engine<_UIntType, __w, __s, __r>::
     seed(result_type __value)
     {
+      // _GLIBCXX_RESOLVE_LIB_DEFECTS
+      // 3809. Is std::subtract_with_carry_engine<uint16_t> supposed to work?
+      // 4014. LWG 3809 changes behavior of some existing code
+      if (__value == 0u)
+       __value = default_seed;
+      else
+       __value %= 2147483563u;
+
       std::linear_congruential_engine<uint_least32_t, 40014u, 0u, 2147483563u>
-       __lcg(__value == 0u ? default_seed : __value);
+       __lcg(__value);

       const size_t __n = (__w + 31) / 32;