Duplicate conditions in case expression should be reported as invalid

[samanpwbb]

mapbox-gl-js version: 0.44.1

Steps to Trigger Behavior

A case expression like follows is valid:

[
  "case",
  ["has", "rank"],
  1,
  ["has", "rank"],
  1,
  0
]

But an expression like following throws an error here:

[
  "match",
  ["get", "rank"],
  100,
  1,
  100,
  1,
  0 
]

Expected Behavior

Both of the above expressions are invalid according to spec expression definitions.

Actual Behavior

Match expression with duplicate condition is invalid, but case expression with duplicate condition is valid.

Adding this extra validation would enable us to lean more on mapbox-gl-js's internal validation rather than rolling our own validation on top.

[jfirebaugh]

This requires an equivalence relation on expressions. Is your expectation that two expressions are considered "the same" if and only if they are "deep equal" JSON structures? So for instance the following is allowed, despite being obviously equivalent in a different sense (same output on all possible inputs):

[
  "case",
  ["has", "rank"],
  1,
  ["all", ["has", "rank"], true],
  1,
  0
]

We could implement this, but I'm not totally convinced of its value, and it would be more expensive than what we do for match, the syntax of which is carefully chosen to make the validation cheap. (For case it would require either O(n^2) deep equality operations, or JSON.stringifying all the input conditionals.)

Do you know of other tools that provide this sort of warning for conditional expressions (if/else if)?

[mourner]

Looks like we're not able to implement this in a performant way at runtime, so we'll have to lean on invalidation in style authoring tools like Studio.