https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111226
Bug ID: 111226
Summary: constexpr doesn't detect change of union to empty
member
Product: gcc
Version: 14.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: nathanieloshead at gmail dot com
Target Milestone: ---
While working on a patch for PR101631, I found that the following code is
currently incorrectly handled by GCC: (https://godbolt.org/z/1YevacMK3)
struct Empty {};
union U {
int x;
Empty e;
};
constexpr int foo() {
U u{ 10 };
u.e = {};
return u.x; // incorrectly accepted, even pre-C++20
}
constexpr auto y = foo();
constexpr Empty bar() {
U u{ 10 };
u.e = {};
return u.e; // incorrectly errors, thinks active member is still 'x'
}
constexpr auto x = bar();
The cause seems to be that the zero-sized trivial assignment is removed in
call.cc (since PR43075) and after constant folding is no longer in the tree
that the constexpr handling machinery receives.
