What I've implemented here deviates from the current proposal text
in the following ways:
- I had to introduce a FunctionArrowPrecedence to capture the parsing
of -> in expression contexts.
- I found it convenient to continue to model the assignment property
explicitly.
- The comparison and casting operators have historically been
non-associative; I have chosen to preserve that, since I don't
think this proposal intended to change it.
- This uses the precedence group names and higherThan/lowerThan
as agreed in discussion.
Lexing for conflict markers is inspired by the way clang handles them with a few
updates of our own. Clang's lexer currently searches for the start of the
conflict marker, attempts to find the divider points, lexes that, then finds the
end. We, unfortunately, cannot be so forgiving because of operator overloads.
Instead, we search for the start and end markers and ignore all text in between.
Even if what is found is not conflict markers, it certainly is not valid Swift either.
