https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107939
Bug ID: 107939
Summary: Rejects use of `extern const` variable in a template
Product: gcc
Version: 13.0
Status: UNCONFIRMED
Keywords: rejects-valid
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: johelegp at gmail dot com
CC: johelegp at gmail dot com
Target Milestone: ---
See https://compiler-explorer.com/z/q3a7e7s1e.
Changing `#if 1` to `#if 0` works, so this can't be due to
> The program is ill-formed, no diagnostic required, if:
> (6.4) a hypothetical instantiation of a template immediately following its
> definition would be ill-formed due to a construct that does not depend on a
> template parameter, or
> -- https://eel.is/c++draft/temp.res.general#6.4
And since all compilers accept it as a non-template, nothing in [expr.const] is
being violated.
```C++
struct Q {
struct P {
const Q* p;
};
int n;
constexpr P operator()(auto) const { return {this}; }
};
extern const Q q;
#if 1
template<int> constexpr auto p = q(0);
static_assert(p<0>.p == &q);
#else
constexpr auto p = q(0);
static_assert(p.p == &q);
#endif
constexpr Q q = {};
```
```
<source>:12:37: error: the value of 'q' is not usable in a constant expression
12 | template<int> constexpr auto p = q(0);
| ^
<source>:9:16: note: 'q' was not declared 'constexpr'
9 | extern const Q q;
| ^
<source>:13:22: error: non-constant condition for static assertion
13 | static_assert(p<0>.p == &q);
| ~~~~~~~^~~~~
Compiler returned: 1
```
