https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94939
Bug ID: 94939
Summary: [9.2.1] invalid code generation in ternary op with
static class member (undefined behaviour nearby?)
Product: gcc
Version: 9.2.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: [email protected]
Target Milestone: ---
I've got the following classes, which are using the type_safe library
(https://github.com/foonathan/type_safe).
struct Old
{
inline constexpr Old();
constexpr Old(std::int64_t v)
: storage(v)
{}
std::int64_t storage = -1;
static const Old magic;
};
constexpr Old::Old() { storage = 0; }
constexpr Old Old::magic = Old(12);
struct OldDerived : Old
{
constexpr OldDerived(Old old)
: Old(old)
{}
constexpr OldDerived(std::int64_t v)
: Old(v)
{}
auto get() { return storage; }
static const OldDerived magic;
};
constexpr OldDerived OldDerived::magic = OldDerived(Old::magic);
struct New : public Old
{
New() = default;
};
template<typename Strongtypedef, typename OldType_>
struct conversions
{
using OldType = OldType_;
const OldType& toOld() const
{
return static_cast<const OldType&>(
static_cast<const Old&>(type_safe::get(static_cast<const
Strongtypedef&>(*this))));
}
};
template<typename ST, typename OT_>
struct one_more_type : conversions<ST, OT_>
{};
struct NewDerived
: type_safe::strong_typedef<NewDerived, New>
, one_more_type<NewDerived, OldDerived>
{
NewDerived() = default;
NewDerived(std::int64_t value)
: strong_typedef(New{{value}})
{}
};
As a colleague already pointed out, conversions::toOld() might have
an illegal cast in it and maybe that is all to this. What I observe is the
following code prints 'VALUE IS 0' (in production: VALUE IS <GARBAGE>).
OldDerived
callee(bool b)
{
NewDerived a = NewDerived{11};
return b ? a.toOld() : OldDerived::magic;
}
__attribute_noinline__ void
caller_magic(bool b)
{
OldDerived value = callee(b);
std::cerr << "VALUE IS " << value.storage << std::endl;
}
void main()
{
caller_magic(true);
}
And if I do not use OldDerived::magic, the following code prints correctly
'VALUE IS 11'.
OldDerived
callee_lit(bool b)
{
NewDerived a = NewDerived{11};
return b ? a.toOld() : OldDerived(12);
}
__attribute_noinline__ void
caller_lit(bool b)
{
OldDerived value = callee_lit(b);
std::cerr << "VALUE IS " << value.storage << std::endl;
}
void main()
{
caller_lit(true);
}
Here is the code, including the parts from type_safe) inside the compiler
explorer: https://godbolt.org/z/TGB8Jg
Is it enough to fix the cast to make sure the code does what one expects?
