I've found very simple case where clang gives wrong diagnosis.

Let's consider smallest possible program:
```
#include <type_traits>

int main() {
    if constexpr (std::is_same_v<int, int>) {}
    else if constexpr (std::is_same_v<double, int>) {}
}
```
clang outputs:
```
error: constexpr if condition is not a constant expression

    else if constexpr (std::is_same_v<double, int>) {}
```

Cause of this is statement:
`if constexpr (std::is_same_v<int, int>)`
any further `if constexpr` statement is not considered as constant expression.