Fix 3845 nullptr cpp

[parrt]

Fixed while doing #3817 Also see @kaby76's fix that I implemented: antlr/grammars-v4#2909

[KvanTTT]

It looks incorrect. Last line is never executed since it return false if any or both arguments are not null. Also, it's not optimal. I suggest fixing in the following way:

Suggested change

	if (lhs == nullptr && rhs == nullptr) return true;
	if (lhs != nullptr || rhs != nullptr) return false;
	return *lhs == *rhs;
	if (lhs == nullptr)
	return rhs == nullptr;
	else
	return rhs != nullptr;
	return *lhs == *rhs;

[parrt]

dang! you're right. I tried to get too clever. grrr...will fix. thanks.

[parrt]

is your line 32 dead code?

[parrt]

maybe this?

if ( lhs == nullptr ) return rhs == nullptr;
if ( rhs == nullptr ) return false; // lhs!=null and rhs==null
return *lhs == *rhs;                // both nonnull

[KvanTTT]

Not dead, it's comparison by dereference (if I remember and understand C++ correctly).

[parrt]

meaning your line 32 is never reached.

[KvanTTT]

Sorry, my first suggestion is also incorrect. The fixed version:

    if (lhs == nullptr)
        return rhs == nullptr;
    else if (rhs == nullptr)
        return false;
    return *lhs == *rhs;

[parrt]

yep trying similar now. seems to work. this one still fails though, but this time with a SEGV #3959 rather than infinite loop.

[KvanTTT]

Yes, it's correct. Didn't get update in time.

[parrt]

pushed the fix. thanks for finding! I updated trace diff for failing test: #3959