https://gcc.gnu.org/bugzilla/show_bug.cgi?id=118227
Bug ID: 118227
Summary: Hard error on unsupported array size when generating
deduction guides for an alias template
Product: gcc
Version: 14.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: bekenn at gmail dot com
Target Milestone: ---
Code: https://godbolt.org/z/jxE9PW13e
#include <span>
template <typename T> using dynamic_span = std::span<T>;
void example()
{
int x[5];
dynamic_span d = x;
static_assert(std::is_same_v<decltype(d), std::span<int,
std::dynamic_extent>>);
}
error: size '18446744073709551615' of array exceeds maximum object size
'9223372036854775807'
The initialization of d prompts the generation of deduction guides for the
alias template dynamic_span; these guides are based on existing deduction
guides for std::span, which include the following:
namespace std {
template<class T, size_t N>
span(T (&)[N]) -> span<T, N>;
}
This deduction guide is transformed into the equivalent of:
template<class T>
dynamic_span(T (&)[std::dynamic_extent]) -> std::span<T,
std::dynamic_extent>;
...and this is where GCC runs into a problem. std::dynamic_extent is
size_t(-1), which is apparently invalid as an array bound, and GCC emits an
error diagnostic to that effect. This is contrary to the rules in the
standard, which essentially states that a substitution failure at this point
simply removes the guide from consideration; [over.match.class.deduct/3]: "If
substitution succeeds, form a function or function template f’ with the
following properties and add it to the set of guides of A: [...]"
This substitution should not result in a hard error or even a warning; instead,
the guide should be silently removed from consideration.
