https://gcc.gnu.org/bugzilla/show_bug.cgi?id=121265
Bug ID: 121265
Summary: CTAD for alias templates
Product: gcc
Version: 15.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: rohamamini at gmail dot com
Target Milestone: ---
#include <type_traits>
template <typename T, typename U = double>
struct Q1
{
T dm1;
U dm2;
};
template <typename T, typename U>
Q1(T, U) -> Q1<T, U>; // f
template <typename X>
using Q1Alias = Q1<X>; // A = B<Arglist>
/*
1) deduce 'Q1<T, U>' from 'Q1<X>'. T = X, U = not deduced
2) Q1(X, U) -> Q1<X, U>; // g
3) template <typename X, typename U>
requires(__is_deducible<Q1<X, U>>)
Q1Alias(X, U) -> Q1<X, U>; // f'
template <typename X>
class AA;
template <typename X>
class AA<Q1Alias<X>> {}; // equivalent to class AA<Q1<X, double>>
template <typename T>
concept __is_deducible = requires { sizeof(AA<T>); };
*/
int main()
{
[[maybe_unused]]auto q1 = Q1Alias(1, 'x'); // i believe should be error
[[maybe_unused]]auto q2 = Q1Alias(1, 2.0); // ok
}
This code compiles with x86‑64 GCC trunk and 15.1 using -std=c++20/23 -Wall
-Wextra, but it fails to compile with x86‑64 Clang trunk. I believe Clang is
correct to reject it, because—as cppreference.com states—“the associated
constraints of f' are the conjunction of the associated constraints of g and a
constraint that is satisfied if and only if the arguments of A are deducible
from the result type,” and in this case those constraints are not met (the
template arguments of the specialization AA<Q1<int, char>> do not match the
template arguments of the partial specialization AA<Q1<X, double>>).
link to godbolt: https://godbolt.org/z/9KqY39bPd
