https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116492
Bug ID: 116492
Summary: inherited constructors in subclass of std::expected
can not be overridden
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: mail at johslarsen dot net
Target Milestone: ---
Created attachment 59007
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=59007&action=edit
preprocessed file
First of all, sorry I subclassed the standard library, and yes, I am perfectly
aware that this is a very bad practice. As such the important part might not be
a fix for this in particular, but that it could shed light on similar problems
elsewhere
The following code snippet compiled with gcc 12.1 (first with std::expected on
godbolt) through 14.2 (and trunk) constructs obj by calling std::expected's
constructor directly instead of the constructor defined in my_expected:
#include <expected>
class my_expected : public std::expected<int, const char*> {
public:
my_expected() : std::expected<int, const char*>::expected(42) {}
using std::expected<int, const char*>::expected;
};
int main() {
my_expected obj;
return *obj != 42;
}
Resulting in *obj being zero (instead of 42), and a non-zero exit value.
This only happens with the using statement, but as far as I understand the C++
documentation the derived constructor should have been used instead:
> [...] if an inherited constructor matches the signature of one of the
> constructors
> of Derived, it is hidden from lookup by the version found in Derived.
> https://en.cppreference.com/w/cpp/language/using_declaration
The same does not happen in clang, and in gcc with similar examples from other
classes I have tried. g++ returns no errors or warnings with -Wall -Wextra and
all the other flags you suggested. The attached preprocessed file was generated
with g++ 14.2.1 20240805
