[BUG] is type for std::optional can return unexpected results

[bluetarpmedia]

Describe the bug
An expression testing if a std::optional is a type can return unexpected results, depending on whether the optional's type is convertible to the target type. (I'm assuming this behaviour is unintended.)

To Reproduce
Run cppfront on the following and then compile with a C++ compiler:

main: () -> int = {
    o: std::optional<int> = 5;
    if o is int {
        std::cout << "int!\n";
    }
    if o is bool {
        std::cout << "bool!\n";
    }
    if o is float {
        std::cout << "float!\n";
    }
    if o is std::string {
        std::cout << "std::string!\n";
    }
    return 0;
}

The output is:

int!
bool!
float!

Repro on Godbolt.

I think this behaviour is due to this cast:

cppfront/include/cpp2util.h

Line 2042 in ddde3d8

if constexpr (requires { static_cast<const T&>(*x);}) {

CC @filipsajdak