Design an expression constraint or form that allows for statically determining all possible output values.

[anandthakker]

It seems that we need to be able to determine statically (i.e., without access to feature data) what all the possible output values are for some expressions:

• text-font - needed because "One of the components of an offline region download is the complete set of glyph range PBFs for all the font stacks used by that region's style"
• icon-image - per chat with @ian29 @aparlato, "we need to know all the icon names referenced by a style in order to check that the icons exist"

In order to do this, we need a way to define an enforceable constraint on expressions that guarantees that guarantees that its possible output values is a subset of the set of string literals used in the expression. (Theoretically possible for the guarantee to be looser, but this is probably good enough.)

[anandthakker]

cc @jfirebaugh

[anandthakker]

Copying over from chat:

@jfirebaugh: It seems like it should be pretty straightforward to write a static analyzer that recursively descends an expression AST to determine potential outputs, with some sort of sentinel value for "unknown"/"dynamic value"
@anandthakker: @jfirebaugh hm, my fear with that approach is that it might be hard for users to understand, without some trial and error, what expressions they could use
@jfirebaugh: @anandthakker Studio can use some stricter but easier to understand rule if it wants to. But this would solve @ian29's need and the offline glyph's need.
@anandthakker: hm yeah, good point. I was thinking in terms of people authoring raw expressions, but as I myself have argued elsewhere, the users doing that are very likely to be expert

[jfirebaugh]

We have a plan for this in mapbox/mapbox-gl-native#9939 (comment).