Bug fixes for parsing associativity

[TysonAndre]

This fixes the most common cases for #19
(there may be other edge cases I haven't considered yet)

This also fixes incorrect precedence for <=>.
It has the same precedence as == -
https://secure.php.net/manual/en/language.operators.precedence.php
has correct information for the <=> operator.

[TysonAndre]

I ran a test of all valid combinations of a subset of binary operators (according to what php-ast said was valid) to check for false positives. (not including assignment by ref)

• This change to Parser.php brought the number of incorrectly parsed ASTs from 311 down to 4. https://github.com/TysonAndre/phan/compare/tests-fallback-only?targetBranch=master

(The 4 remaining edge cases include $x ?? $y ?: $z and $x ?: $y ?: $z, which were pre-existing. The other two were nonsense with instanceof that shouldn't occur in valid code.)

The number of errors in the validation tests is still 3, and I think those were pre-existing (for the symfony edge cases)

[TysonAndre]

Is there anything else that should be done in this PR?

[roblourens]

Sorry, I will look at it soon. Been busy/on vacation.

[roblourens]

This looks good but I don't understand why $x == $y <=> $z; is a syntax error but $x < $a <=> $b; is not? I must be missing something.

[TysonAndre]

This is part of the standard that most programmers normally don't need to think about.

The PHP manual mentions this (replace == with <=> in that sentence). See http://php.net/manual/en/language.operators.precedence.php

Operators of equal precedence that are non-associative cannot be used next to each other, for example 1 < 2 > 1 is illegal in PHP. The expression 1 <= 1 == 1 on the other hand is legal, because the == operator has lesser precedence than the <= operator.

The below two groups have the same precedences within the groups:

• (non-associative) < <= > >=
• (non-associative) == != === !== <> <=>

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I'm assuming this is so that expressions such as X < Y == EXPECTED_BOOL_VAL will work, but ambiguous expressions would be detected as a parse error (e.g. X == Y == Z)

php > var_export(0 < 3 == true);
true

[roblourens]

That's kind of crazy but makes sense, thanks!