https://gcc.gnu.org/bugzilla/show_bug.cgi?id=117772
Bug ID: 117772
Summary: constrained auto in decl-specifier accepted for
function return type despite trailing-return-type
Product: gcc
Version: 14.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: ing.russomauro at gmail dot com
Target Milestone: ---
C++20 standard, in ยง9.3.3.5 [dcl.fct], p2 reads
In a declaration T D where D has the form
D1 .... trailing-return-type
..., T shall be the single type-specifier auto.
....
Therefore, the following program should not be accepted:
(see: https://godbolt.org/z/E35efdTjo)
#include <concepts>
template<typename T>
concept Cni = !std::integral<T>;
Cni auto foo() -> std::same_as<void> auto{};
std::same_as<void> auto bar() -> Cni auto{};
template<typename T>
concept Ct = true;
Ct auto baz() -> Ct auto{}; // clang rejects this, too
int main(){}
Indeed, clang 19 rejects it, even in case of same constraint name:
"error: function with trailing return type must specify return type 'auto', not
'Ct auto'"
However, MVSC v19 also accepts (not tried with higher warning settings).
I see a similar bug being Bug 80273 (cv-qualification),
but also likely only another case ('auto' twice) compared to Bug 100589.
Please, check whether this is definitively a new bug or not.
