[FIX] add additional parens for CPP2_TYPEOF macro used in inspect

[filipsajdak]

The current implementation of cppfront is using constexpr ifs with requires expressions and the CPP2_TYPEOF macro to eliminate the compilation of alternatives that did not match in inspect expression. It suffers from issues with using macros and commas.

The below code:

template <int i, int j>
auto calc() {
    return i + j;
}

fun: (v : _) -> int = {
    return inspect v -> int {
        is int  = calc<1,2>();
        is _ = 0;
    };
}

main: () -> int = {
    return fun(42);
}

generates to (skipping boilerplate):

[[nodiscard]] auto fun(auto const& v) -> int{
    return [&] () -> int { auto&& __expr = v;
        if (cpp2::is<int>(__expr)) { if constexpr( requires{calc<1,2>();} ) if constexpr( std::is_convertible_v<CPP2_TYPEOF(calc<1,2>()),int> ) return calc<1,2>(); else return int{}; else return int{}; }
        else return 0; }
    ()
; }

[[nodiscard]] auto main() -> int{
    return fun(42); 
}

The problematic part is CPP2_TYPEOF(calc<1,2>()) that fails to compile (because of the coma) with the error:

tests/inspect_return_template.cpp2:12:132: error: too many arguments provided to function-like macro invocation
        if (cpp2::is<int>(__expr)) { if constexpr( requires{calc<1,2>();} ) if constexpr( std::is_convertible_v<CPP2_TYPEOF(calc<1,2>()),int> ) return calc<1,2>(); else return int{}; else return int{}; }
                                                                                                                                   ^
include/cpp2util.h:209:9: note: macro 'CPP2_TYPEOF' defined here
#define CPP2_TYPEOF(x)  std::remove_cvref_t<decltype(x)>
        ^
tests/inspect_return_template.cpp2:12:113: error: use of undeclared identifier 'CPP2_TYPEOF'
        if (cpp2::is<int>(__expr)) { if constexpr( requires{calc<1,2>();} ) if constexpr( std::is_convertible_v<CPP2_TYPEOF(calc<1,2>()),int> ) return calc<1,2>(); else return int{}; else return int{}; }
                                                                                                                ^
error: constexpr if condition is not a constant expression
tests/inspect_return_template.cpp2:18:12: note: in instantiation of function template specialization 'fun<int>' requested here
    return fun(42); 
           ^
3 errors generated.

This fix adds extra parens and produce:

CPP2_TYPEOF((calc<1,2>()))

Closes: #147

[hsutter]

Looks good! I'll merge this then add the new test case above to /regression-tests.