https://gcc.gnu.org/bugzilla/show_bug.cgi?id=127835
Bug ID: 127835
Summary: unused expression result makes compiler allow invalid
code in constant evaluation
Product: gcc
Version: 17.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: irvsrc at gmail dot com
Target Milestone: ---
The following invalid code compiles without errors:
```
struct ty {
int n = 0;
auto consteval fn() {
n + 1;
}
};
auto main() -> int {
auto v = ty{};
v.fn();
}
```
However, the call to `v.fn()` is ill-formed because:
1) The call to consteval function `ty::fn` is not a constant expression,
because the read of non-constexpr variable `v` is not allowed in a constant
evaluation.
2) Even if `v` is made a constexpr variable, then the `this` argument to member
function `fn` has type `const ty`, however the function is not const-qualified.
I believe this issue may be due to constant evaluation rules not being applied
to unused expression results.
This also occurs across many other GCC and C++ versions from what I have
tested.
GCC (trunk)
-std=c++26
