[Cedar 3.0] Desugaring of greater-than operator changes evaluation order

[mattmccutchen-amazon]

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Bug Category

Cedar Parser

Describe the bug

The CST-AST conversion desugars a > b into b < a and a >= b into b <= a. That changes the evaluation order, which is visible to users via its effect on which error gets reported if both a and b raise an error and possibly via performance differences as well. The effect on error reporting could be confusing to users: Cedar's evaluation order is generally left to right, so if an expression a op b raises an error from b, I think an average user might think that means a evaluated successfully and might get led down a wrong path during debugging.

Expected behavior

When an expression of the form a > b is evaluated, a should be evaluated before b, as with most other Cedar binary operators.

Reproduction steps

This can be reproduced using the cedar command-line tool:

% cedar evaluate 'context.nosuch < (4294967296 * 4294967296)'

error while evaluating the expression: record does not have the required attribute: nosuch

% cedar evaluate 'context.nosuch > (4294967296 * 4294967296)'

error while evaluating the expression: integer overflow while attempting to multiply 4294967296 by 4294967296

In both commands of this example, both the left and the right operand raise an error. The < operator propagates the error from the left operand, while the > operator propagates the error from the right operand.

Code Snippet

Included in the "reproduction steps".

Log output

Included in the "reproduction steps".

Additional configuration

No response

Operating System

Linux

Additional information and screenshots

A possible solution that doesn't require adding any more operators to the AST: change the desugaring of a > b to !(a <= b), and change the desugaring of a >= b to !(a < b).