https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102529
Bug ID: 102529
Summary: ctad for aliases fails in the presence of constraints
Product: gcc
Version: 12.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: barry.revzin at gmail dot com
Target Milestone: ---
Here's an example:
template <typename T>
struct C {
template <typename U>
C(U);
};
template <typename U>
C(U) -> C<U*>;
template <typename T>
requires true
using A = C<T>;
C ok(1); // ok, a is a C<int*>
A bad(2); // fails
The provided error on gcc 11.2 (and trunk as of this writing) is:
<source>:15:8: error: class template argument deduction failed:
15 | A bad(2); // fails
| ^
<source>:15:8: error: no matching function for call to 'C(int)'
<source>:2:8: note: candidate: 'template<class T> C(C<T>)-> C<T> requires
__is_same(A<T>, C<T>)'
2 | struct C {
| ^
<source>:2:8: note: template argument deduction/substitution failed:
<source>:15:8: note: mismatched types 'C<T>' and 'int'
15 | A bad(2); // fails
| ^
<source>:4:5: note: candidate: 'template<class T, class U> C(U)-> C<T> requires
__is_same(A<T>, C<T>)'
4 | C(U);
| ^
<source>:4:5: note: template argument deduction/substitution failed:
<source>:15:8: note: couldn't deduce template parameter 'T'
15 | A bad(2); // fails
| ^
If you drop the "requires true" part of the alias template A, then this works.
As far as I understand the rules, this should work. MSVC accepts the example.
clang does not yet implement class template argument deduction for alias
templates.
