https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108897
Bug ID: 108897
Summary: Comparing pointer to field in rvalue class is not
considered constant expression
Product: gcc
Version: 13.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: danakj at orodu dot net
Target Milestone: ---
This reproduces on godbolt in GCC 10, 11, 12, and trunk.
A pointer to a field of a structure that is constructed as an rvalue temporary
object can not be compared in a constant expression.
```
struct S {
int i;
int *p = &i;
constexpr const int* ptr() const { return &i; }
};
// Not constant expression in GCC.
static_assert(S{2}.ptr() != nullptr);
static_assert(S{2}.p != nullptr);
// Is a constant expression.
static_assert(*S{2}.ptr() == 2);
static_assert(*S{2}.p == 2);
static_assert([]() constexpr {
S r(2);
// Is a constant expression.
return r.ptr() != nullptr && r.p != nullptr;
}());
```
Errors:
<source>:8:26: error: non-constant condition for static assertion
8 | static_assert(S{2}.ptr() != nullptr);
| ~~~~~~~~~~~^~~~~~~~~~
<source>:8:26: error: '((&<anonymous>.S::i) != 0)' is not a constant expression
<source>:9:22: error: non-constant condition for static assertion
9 | static_assert(S{2}.p != nullptr);
| ~~~~~~~^~~~~~~~~~
<source>:9:22: error: '((&<anonymous>.S::i) != 0)' is not a constant expression
- Generates errors for comparing `ptr()` or `p` to nullptr.
- Dereferencing the pointer does not generate an error.
- If the object S is stored as an lvalue, then the pointer comparison is a
constant expression.
Godbolt: https://godbolt.org/z/P9GGKeb3Y
