[BUG] can not override a virtual function to pure virtual

[Xeverous]

Describe the bug cppfront does not allow to override a function into pure virtual. Whether base type function is pure or not has no impact.

To Reproduce sample code:

base: type =
{
    d: (virtual move this);
    f: (virtual this) -> int = 1;
}

derived: type =
{
    this: base = ();
    f: (override this) -> int;
}

expected result - something like this:

class base
{
public:
    virtual ~base() = default;
    virtual int f() { return 1; } 
};

class derived : public base
{
public:
    int f() override = 0;
};

actual result: main.cpp2(10,5): error: a function must have a body ('=' initializer), unless it is virtual (has a 'virtual this' parameter) or is defaultable (operator== or operator<=>)

Command lines

cppfront main.cpp2 -p, built from commit dc3758a74d2ab815b7d4c5a19a7635e34e379ef1

Additional context

Conflicting documentation - https://hsutter.github.io/cppfront/cpp2/types/ says:

A pure virtual function is a function with a virtual this or override this parameter and no body.

[gregmarr]

Probably need to add a check for override here. https://github.com/hsutter/cppfront/blob/dc3758a74d2ab815b7d4c5a19a7635e34e379ef1/source/sema.h#L1636-L1649

I wasn't even aware that this was a thing. I've never seen a pure virtual override, but I can see where it can be useful.