| Issue |
136464
|
| Summary |
Clang checks impossible cast
|
| Labels |
clang
|
| Assignees |
|
| Reporter |
vzze
|
### Issue
Clang tries to check if cast of `const char*` is ill-formed even though the context of the cast is reduced to only `int` by `if constexpr`
<details>
<summary>Code</summary>
```c++
// main.cc
#include <type_traits>
#include <variant>
int main() {
using type = std::variant<int, const char*>;
type a = 12;
type b = 42;
int res = std::visit([&](auto a1) -> int {
return std::visit([&](auto b1) -> int {
if constexpr(
std::is_same_v<std::decay_t<decltype(a1)>, std::decay_t<decltype(b1)>> &&
std::is_same_v<std::decay_t<decltype(a1)>, int>
)
return int(a1) + int(b1);
else
throw "Wrong type!";
}, b);
}, a);
}
```
<details>
<summary>Unix Error</summary>
```
main.cc:16:24: error: cast from pointer to smaller type 'int' loses information
16 | return int(a1) + int(b1);
| ^~~~~~~
main.cc:11:47: note: while substituting into a lambda _expression_ here
11 | return std::visit([&](auto b1) -> int {
| ^
/usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/14.2.1/../../../../include/c++/14.2.1/bits/invoke.h:61:14: note: in instantiation of function template specialization 'main():
:(anonymous class)::operator()<const char *>' requested here
61 | { return std::forward<_Fn>(__f)(std::forward<_Args>(__args)...); }
| ^
/usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/14.2.1/../../../../include/c++/14.2.1/bits/invoke.h:96:19: note: in instantiation of function template specialization 'std::__
invoke_impl<int, (lambda at main.cc:10:26), const char *&>' requested here
96 | return std::__invoke_impl<__type>(__tag{}, std::forward<_Callable>(__fn),
| ^
/usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/14.2.1/../../../../include/c++/14.2.1/variant:1060:16: note: in instantiation of function template specialization 'std::__invo
ke<(lambda at main.cc:10:26), const char *&>' requested here
1060 | return std::__invoke(std::forward<_Visitor>(__visitor),
| ^
/usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/14.2.1/../../../../include/c++/14.2.1/variant:1820:5: note: in instantiation of member function 'std::__detail::__variant::__g
en_vtable_impl<std::__detail::__variant::_Multi_array<std::__detail::__variant::__deduce_visit_result<int> (*)((lambda at main.cc:10:26) &&, std::variant<int, const cha
r *> &)>, std::integer_sequence<unsigned long, 1>>::__visit_invoke' requested here
1820 | _GLIBCXX_VISIT_CASE(1)
| ^
/usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/14.2.1/../../../../include/c++/14.2.1/variant:1811:4: note: expanded from macro '_GLIBCXX_VISIT_CASE'
1811 | __visit_invoke(std::forward<_Visitor>(__visitor), \
| ^
/usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/14.2.1/../../../../include/c++/14.2.1/variant:1882:18: note: in instantiation of function template specialization 'std::__do_v
isit<std::__detail::__variant::__deduce_visit_result<int>, (lambda at main.cc:10:26), std::variant<int, const char *> &>' requested here
1882 | return std::__do_visit<_Tag>(
| ^
main.cc:10:20: note: in instantiation of function template specialization 'std::visit<(lambda at main.cc:10:26), std::variant<int, const char *> &>' requested here
10 | int res = std::visit([&](auto a1) -> int {
| ^
1 error generated.
```
</details>
<details>
<summary> Win11 Error </summary>
```
.\main.cc:16:24: warning: cast to smaller integer type 'int' from 'const char *' [-Wpointer-to-int-cast]
16 | return int(a1) + int(b1);
| ^~~~~~~
.\main.cc:11:47: note: while substituting into a lambda _expression_ here
11 | return std::visit([&](auto b1) -> int {
| ^
C:\Program Files\Microsoft Visual Studio\2022\Community\VC\Tools\MSVC\14.43.34808\include\type_traits:1706:16: note: in
instantiation of function template specialization 'main()::(anonymous class)::operator()<const char *>' requested
here
1706 | return static_cast<_Callable&&>(_Obj)(static_cast<_Ty1&&>(_Arg1), static_cast<_Types2&&>(_Args2)...);
| ^
C:\Program Files\Microsoft Visual Studio\2022\Community\VC\Tools\MSVC\14.43.34808\include\variant:1422:25: note: in
instantiation of function template specialization 'std::invoke<(lambda at ./main.cc:10:26), const char *&>'
requested here
1422 | return _STD invoke(static_cast<_Callable&&>(_Obj),
| ^
C:\Program Files\Microsoft Visual Studio\2022\Community\VC\Tools\MSVC\14.43.34808\include\variant:1492:23: note: in
instantiation of function template specialization 'std::_Variant_dispatcher<std::integer_sequence<unsigned long
long, 2>>::_Dispatch2<int, (lambda at ./main.cc:10:26), std::variant<int, const char *> &, false>' requested here
1492 | _STL_STAMP(4, _STL_VISIT_STAMP);
| ^
C:\Program Files\Microsoft Visual Studio\2022\Community\VC\Tools\MSVC\14.43.34808\include\variant:1546:49: note: in
instantiation of function template specialization 'std::_Visit_strategy<1>::_Visit2<int,
std::_Meta_list<std::integer_sequence<unsigned long long, 0>, std::integer_sequence<unsigned long long, 1>,
std::integer_sequence<unsigned long long, 2>>, (lambda at ./main.cc:10:26), std::variant<int, const char *> &>'
requested here
1546 | return _Visit_strategy<_Strategy>::template _Visit2<_Ret, _ListOfIndexVectors>(
| ^
C:\Program Files\Microsoft Visual Studio\2022\Community\VC\Tools\MSVC\14.43.34808\include\variant:1591:17: note: in
instantiation of function template specialization 'std::_Visit_impl<3ULL, int,
std::_Meta_list<std::integer_sequence<unsigned long long, 0>, std::integer_sequence<unsigned long long, 1>,
std::integer_sequence<unsigned long long, 2>>, (lambda at ./main.cc:10:26), std::variant<int, const char *> &>'
requested here
1591 | return _STD _Visit_impl<_Size, _Ret, _ListOfIndexVectors>(
| ^
.\main.cc:10:20: note: in instantiation of function template specialization 'std::visit<(lambda at ./main.cc:10:26),
std::variant<int, const char *> &, void>' requested here
10 | int res = std::visit([&](auto a1) -> int {
| ^
1 warning generated.
```
</details>
</details>
### Test Reproduction
* tested on both `linux` and `win11`
```
clang++ main.cc -std=c++17 -o main
```
### Clang Version
#### Arch Linux
```
clang version 19.1.7
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin
```
#### Windows 11
```
clang version 20.1.3
Target: x86_64-pc-windows-msvc
Thread model: posix
InstalledDir: C:\Users\****\scoop\apps\llvm\20.1.3\bin
```
### Additional information
* compiles just fine with `gcc`
```
g++ main.cc -std=c++17 -o main
```
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