test for assignment + logical operators precedence

[heartsentwined]

This PR adds (failing) test cases demonstrating an assignment operator vs logical operator precedence issue / behavior.

The issue

Compile errors first:

Syntax error on line 1, column 21: unexpected '=' (\u003D)
1 : true if true && foo = 'bar'
^ :~~~~~~~~~~~~~~~~~~~~~^
2 :

(foo should be assigned the value of 'bar'.)

Syntax error on line 1, column 21: unexpected '=' (\u003D)
1 : true if true && foo = 'bar' && true
^ :~~~~~~~~~~~~~~~~~~~~~^
2 :

(foo should be assigned the value of true.)

Similar errors tested for and, || and or.

All compile fine in the original coffeescript compiler.

For compleness, this works fine:

true if foo = 'bar' && true

Existing workaround

Adding parens to group the assignment foo = 'bar' fixes all cases:

true if true && (foo = 'bar')
true if true && (foo = 'bar' && true)

Syntax discussion

I believe that this is an operator precedence issue. While logical operators should, of course, normally take precedence over assignment operators, this should only happen when the logical operator appears at the right hand side of the assignment operator.

Given

true || foo = 'bar' && true

While the && operator taking precedence over the assignment operator makes sense, the case of the || operator would produce this semantically invalid parsing:

(true) = (true)

Since all logical operators will produce a boolean primitive, and that boolean primitives cannot be the receiver of the assignment operator, an exception should be made to change the operator precedence on such cases, producing:

(true) || (foo = true)

I'm sorry but I am not familiar with pegjs, so I can only produce a test case in this PR accompanied by this description.

[michaelficarra]

Thanks for the analysis, but this is already known. See #43. We're still unsure about this syntax.

[heartsentwined]

Thanks for the pointer, I'll join discussions over there.