https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91911
Bug ID: 91911
Summary: Strange interaction between CTAD and decltype
Product: gcc
Version: 10.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: jeremy at jeremyms dot com
Target Milestone: ---
See example program below. Works in clang and MSVC. Fails in GCC 7.1 through
trunk.
#include <type_traits>
#include <vector>
#include <utility>
template <typename T>
struct span {
using value_type = T;
span(std::vector<T> &x) {}
};
template <typename X>
using SpanType = decltype(span(std::declval<X>()));
// Compilation succeeds if we change line below to #if 1
#if 0
template <typename X>
using ConstSpanType =
span<const typename decltype(span(std::declval<X>()))::value_type>;
#else
template <typename X>
using ConstSpanType = span<const typename SpanType<X>::value_type>;
#endif
static_assert(
std::is_same_v<ConstSpanType<std::vector<int>&>, span<const int>>);
Error:
source>: In substitution of 'template<class X> using ConstSpanType = span<const
typename decltype ((span<...auto...>)(declval<X>()))::value_type> [with X =
std::vector<int>&]':
<source>:25:51: required from here
<source>:21:7: error: 'std::vector<int>&' is not a class, struct, or union type
21 | using ConstSpanType = span<const typename SpanType<X>::value_type>;
| ^~~~~~~~~~~~~
<source>:25:10: error: template argument 1 is invalid
25 | std::is_same_v<ConstSpanType<std::vector<int>&>, span<const int>>);
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Compiler returned: 1
https://godbolt.org/z/ir4U26
