https://gcc.gnu.org/bugzilla/show_bug.cgi?id=125900
Bug ID: 125900
Summary: A splice expression in a static_cast expression within
a template-for statement causes an unexpected error:
"consteval-only expressions are only allowed in a
constant-evaluated context"
Product: gcc
Version: 17.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: nishuangcheng at gmail dot com
Target Milestone: ---
### Environment
- Compiler: GCC 16/17
- Flags: -std=c++26 -freflection
### Description
A splice expression in a `static_cast` expression within a `template for`
statement causes an unexpected error: "consteval-only expressions are only
allowed in a constant-evaluated context".
If a splice expression is used outside a `template for` statement or directly
within a `template for` statement, the code compiles correctly.
Besides, the experimental reflection branch of Clang accepts all the cases.
See the [example](https://godbolt.org/z/1GsfT4Gxq) below.
```cpp
#include <meta>
#include <print>
struct S {
float f;
};
int main() {
S s{.f = 1.5f};
constexpr auto member{^^S::f};
std::println("{}", s.[:member:]); // ok
std::println("{}", static_cast<int>(s.[:member:])); // ok
constexpr auto access_context{std::meta::access_context::current()};
template for (constexpr auto member :
std::define_static_array(nonstatic_data_members_of(^^S,
access_context))) {
std::println("{}", s.[:member:]); // ok
std::println("{}", static_cast<int>(s.[:member:])); // error with GCC,
ok with Clang
}
}
```
