| Issue |
184562
|
| Summary |
[Regression][Concepts] Clang 22 regression: concept default NTTP (g = a::e<f>::h) appears to be reused across instantiations, breaking || short-circuit in constraint checking
|
| Labels |
clang
|
| Assignees |
|
| Reporter |
craffael
|
Reproducer obtained with CReduce (C++20, no headers):
```
namespace a {
template <typename b> struct c { using d = b; };
template <typename b> using e = c<b>::d;
} // namespace a
template <class f, char g = a::e<f>::h>
concept i = g < 3 || requires { typename f::template j<3>; };
struct k { static constexpr char h = 2; };
struct l {
static constexpr char h = 3;
template <char> using j = k;
};
template <i f> struct m {
using n = m<typename f::template j<1>>;
};
m<l> n;
```
## How to build
- Fails: clang++-22 -std=c++20 -c repro.cpp
- Works: clang++-21 -std=c++20 -c repro.cpp
- Works: g++-15 -std=c++20 -c repro.cpp
## Actual behavior (clang++-22)
```
<source>:17:13: error: constraints not satisfied for class template 'm' [with f = typename l::template j<1>]
17 | using n = m<typename f::template j<1>>;
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
<source>:19:6: note: in instantiation of template class 'm<l>' requested here
19 | m<l> n;
| ^
<source>:16:11: note: because 'typename l::template j<1>' (aka 'k') does not satisfy 'i'
16 | template <i f> struct m {
| ^
<source>:8:13: note: because ''\x03' < 3' (3 < 3) evaluated to false
8 | concept i = g < 3 || requires { typename f::template j<3>; };
| ^
<source>:8:54: note: and 'typename f::template j<3>' would be invalid: no member named 'j' in 'k'
8 | concept i = g < 3 || requires { typename f::template j<3>; };
| ^
```
## Expected behavior
This should compile.
## Notes / workaround
- Replacing the default with char g = f::h makes clang++-22 accept the code, so the issue seems tied to the alias chain a::e<f> = c<f>::d in the default NTTP.
- static_assert(i<k>); by itself passes on clang++-22; the failure only appears in the nested constrained instantiation (m<l> → m<k>).
## Versions
- Failing: Ubuntu clang version 22.1.1 (Target: x86_64-pc-linux-gnu)
- Working: Ubuntu clang version 21.1.8; g++-15 (GCC) 15.2.1
- OS: Ubuntu 24.04.2 LTS
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