https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89367
Bug ID: 89367
Summary: Constexpr expression is not constexpr in template, but
is constexpr in non-template.
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: frank.secilia at gmail dot com
Target Milestone: ---
Created attachment 45737
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=45737&action=edit
minimal preprocessed test case
Morning, folks!
I'm trying to make compile-time type ids. It's based on the fact that per-type
static instances have unique addresses. I'm using those addresses as the type
id.
Originally, I figured this wouldn't work because taking the address of
something couldn't be constexpr, but the address of a variable is known at
compile time, so I gave it a try. g++ accepts it for a non-template type, but
not if the type is a template.
Attached is a preprocessed minimal test case, but here is something slightly
more verbose with a few comments:
// Concrete type with unique, per-type constexpr id.
struct type_1
{
static constexpr char const unique_address{};
static constexpr void const* const id{&unique_address};
};
constexpr char const type_1::unique_address;
constexpr void const* const type_1::id;
// Another concrete type with same.
struct type_2
{
static constexpr char const unique_address{};
static constexpr void const* const id{&unique_address};
};
constexpr char const type_2::unique_address;
constexpr void const* const type_2::id;
// Types have different ids and these evaluate at compile time, as expected.
static_assert(type_1::id != type_2::id);
// Same, but in a template.
template <typename wrapped>
struct type_id
{
static constexpr char const unique_address{};
static constexpr void const* const id{&unique_address};
};
template <typename wrapped>
constexpr char const type_id<wrapped>::unique_address;
template <typename wrapped>
constexpr void const* const type_id<wrapped>::id;
// This does not compile.
static_assert(type_id<int>::id != type_id<float>::id);
The reasons I think this is a bug is because the only difference between the
succeeding and failing case is that the failing case is templated, and because
this works on clang.
I posted about this on stack overflow here
https://stackoverflow.com/questions/43955536/constexpr-unique-id-compiles-with-clang-but-not-with-gcc.
The simpler example at the bottom is missing a const on the out-of-line
definitions, but the behavior is the same with or without them.
