[Bug c++/85749] New: Possible -Wsign-conversion false negative with std::default_random_engine

[martingalvan at sourceware dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85749

            Bug ID: 85749
           Summary: Possible -Wsign-conversion false negative with
                    std::default_random_engine
           Product: gcc
           Version: 7.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: martingalvan at sourceware dot org
  Target Milestone: ---

I have the following code:

#include <chrono>
#include <random>

int main()
{
    const auto seed =
std::chrono::system_clock::now().time_since_epoch().count();
    std::default_random_engine generator(seed);

    generator.seed(seed);

    return 0;
}

If I build this using g++ -std=c++17 -Wsign-conversion -o random random.cpp, I
get the following warning:

random.cpp: In function ‘int main()’:
random.cpp:9:24: warning: conversion to ‘std::linear_congruential_engine<long
unsigned int, 16807, 0, 2147483647>::result_type {aka long unsigned int}’ from
‘long int’ may change the sign of the result [-Wsign-conversion]
     generator.seed(seed);

However, AFAIK the std::default_random_engine constructor also takes a
result_type (which should be unsigned). Therefore, the warning should also come
up when constructing 'generator'.

$ g++ -v
Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/7/lto-wrapper
OFFLOAD_TARGET_NAMES=nvptx-none
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu
7.2.0-8ubuntu3.2' --with-bugurl=file:///usr/share/doc/gcc-7/README.Bugs
--enable-languages=c,ada,c++,go,brig,d,fortran,objc,obj-c++ --prefix=/usr
--with-gcc-major-version-only --program-suffix=-7
--program-prefix=x86_64-linux-gnu- --enable-shared --enable-linker-build-id
--libexecdir=/usr/lib --without-included-gettext --enable-threads=posix
--libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu
--enable-libstdcxx-debug --enable-libstdcxx-time=yes
--with-default-libstdcxx-abi=new --enable-gnu-unique-object
--disable-vtable-verify --enable-libmpx --enable-plugin --enable-default-pie
--with-system-zlib --with-target-system-zlib --enable-objc-gc=auto
--enable-multiarch --disable-werror --with-arch-32=i686 --with-abi=m64
--with-multilib-list=m32,m64,mx32 --enable-multilib --with-tune=generic
--enable-offload-targets=nvptx-none --without-cuda-driver
--enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu
--target=x86_64-linux-gnu
Thread model: posix
gcc version 7.2.0 (Ubuntu 7.2.0-8ubuntu3.2)