https://gcc.gnu.org/bugzilla/show_bug.cgi?id=118719
Bug ID: 118719
Summary: [14/15 regression] Spurious -Wc++20-extensions warning
emitted when capturing a structured binding
Product: gcc
Version: 15.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: dangelog at gmail dot com
Target Milestone: ---
Hi,
This is a spin-off of bug 85889, comment 8 .
GCC 15 added a compatibility warning when capturing a structured binding in
lambda's capture list, in pre-C++20 modes. The problem is that the warning is
also emitted when a structured binding is used in an init-capture's
initializer, not just in a simple-capture:
https://gcc.godbolt.org/z/TnzWs8xne
int main()
{
int a[] = { 42 };
auto [x] = a;
[x](){}; // 1
[x=x](){}; // 2
[test = x](){}; // 3
[test = x * 2](){}; // 4
}
> <source>: In function 'int main()':
> <source>:7:6: warning: captured structured bindings are a C++20 extension
> [-Wc++20-extensions]
> 7 | [x](){}; // 1
> | ^
> <source>:5:11: note: declared here
> 5 | auto [x] = a;
> | ^
> <source>:8:8: warning: captured structured bindings are a C++20 extension
> [-Wc++20-extensions]
> 8 | [x=x](){}; // 2
> | ^
> <source>:5:11: note: declared here
> 5 | auto [x] = a;
> | ^
> <source>:9:13: warning: captured structured bindings are a C++20 extension
> [-Wc++20-extensions]
> 9 | [test = x](){}; // 3
> | ^
> <source>:5:11: note: declared here
> 5 | auto [x] = a;
> | ^
> Compiler returned: 0
(1) is OK, but (2) and (3) shouldn't warn. (4) doesn't warn.
AFAICS, the code should be fine, as init-captures don't impose anything here?
* In C++17 (N4659), [expr.prim.lambda.capture]/6: "An init-capture behaves as
if it declares and explicitly captures a variable of the form “auto
init-capture ;” whose declarative region is the lambda-expression's
compound-statement"
(https://timsong-cpp.github.io/cppwp/n4659/expr.prim.lambda.capture#6)
* The current draft says: "An init-capture without ellipsis behaves as if it
declares and explicitly captures a variable of the form “auto init-capture ;”"
(https://eel.is/c++draft/expr.prim.lambda.capture#6)
For comparison, Clang doesn't warn on (2) and (3).
