fix($parse): make equality and relational operators left-associative

[teropa]

Request Type: bug

How to reproduce: Evaluate "1 === 1 === true". Expected: true. Actual: false.

Component(s): $parse

Impact: small

Complexity: small

This issue is related to:

Detailed Description:

This fixes an associativity issue in expressions that use multiple
equality or relational operators back to back without grouping.

Previously, these expressions were right-associative, which may
cause confusion for users who expect them to behave identically
to JavaScript expressions.

An expression like "a === b === c" was evaluated as "a === (b === c)".
Now it is evaluated as it is in JavaScript: "(a === b) === c".

Breaks expressions that have relied on right-associativity.

Other Comments:

[mary-poppins]

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[lgalfaso]

Hi @teropa, this is a breaking change, can you please state so in the commit log?

Thinking out loud: should the behavior of = also change? This is so a = b = null is also possible

[caitp]

people shouldn't even be doing expressions like this in angular expressions, but @lgalfaso is right, this is a breaking change, and that might make it difficult to backport into 1.2.

@lgalfaso I don't think assignment needs to be left-associative, you'd expect the right-most assignment operation to occur first, before the left-hand ones. But really, that kind of stuff shouldn't even be happening in angular expressions :(

[teropa]

Added a mention of the breakage to the commit message.

Agree with @caitp that people shouldn't be doing this in Angular expressions, nor in JavaScript. In any case, this looks to me like a deviation from JavaScript's behavior for no apparent reason.

Assignment is right-associative in both JavaScript and ng expressions.

[lgalfaso]

I know assignment is right-associative, but running

scope.$eval('a=b=123');

throws

Error: [$parse:syntax] Syntax Error: Token '=' is an unexpected token at column 4 of the expression [a=b=123] starting at [=123].

One way or another I do see this other issue as out of the scope of this PR

[lgalfaso]

closing this in favor of #10592