| Issue |
63837
|
| Summary |
Concept-constrained non-type template parameter pack in a member template of a class template causes a crash
|
| Labels |
|
| Assignees |
|
| Reporter |
jiriklepl
|
Simple code to reproduce the bug [https://godbolt.org/z/97PKnxeqP](https://godbolt.org/z/97PKnxeqP):
```cpp
template<class T>
concept IsFoo = true;
template<class>
struct Struct {
template<IsFoo auto... xs>
void method() {}
};
template void Struct<void>::method<>();
```
The crash happens on all currently available versions capable of interpreting constraints.
The actual template parameters of the template installation do not matter, I have chosen the simplest ones.
The code is interpreted correctly by the latest [GCC](https://godbolt.org/z/z7McMjzrj) and [MSVC](https://godbolt.org/z/968W13hEq).
---
A very similar code with equivalent semantics that doesn't cause the crash [https://godbolt.org/z/efG5cnqMn](https://godbolt.org/z/efG5cnqMn):
```cpp
template<class T>
concept IsFoo = true;
template<class>
struct Struct {
template<auto... xs> requires (... && IsFoo<decltype(xs)>)
void method() {}
};
template void Struct<void>::method<>();
```
The above example could lead to an assumption that the crash is caused by `IsFoo auto` simply appearing in a parameter pack, but that does not seem to be the case as the following simplification of the code, again, is interpreted as expected [https://godbolt.org/z/eYE995q54](https://godbolt.org/z/eYE995q54):
```cpp
template<class T>
concept IsFoo = true;
struct Struct {
template<IsFoo auto... xs>
void method() {}
};
template void Struct::method<>();
```
---
The following example demonstrates that bug is not limited to methods [https://godbolt.org/z/GxKd85r7c](https://godbolt.org/z/GxKd85r7c):
```cpp
template<class T>
concept IsFoo = true;
template<class>
struct Struct {
template<IsFoo auto... xs>
static constexpr int field = 0;
};
template constexpr int Struct<void>::field<>;
```
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