Redefinition of a duplicate syntax binding should be an error

[disnet]

Currently a duplicate syntax declaration in the same scope will override the previous binding:

syntax m = ctx => #`first`;
syntax m = ctx => #`second`;
m; // second

It should be following let/const binding semantics and throw an early error.

[gabejohnson]

Is this the spot?

https://github.com/mozilla/sweet.js/blob/master/src/enforester.js#L1804

[gabejohnson]

nm. Didn't see you had self-assigned

[disnet]

It's actually here: https://github.com/mozilla/sweet.js/blob/master/src/token-expander.js#L23-L34

But its a bit subtle so I need to think through it.

[gabejohnson]

This does throw an error now

Error: Scopeset {outsideEdge_1, insideEdge0_2} has ambiguous subsets {outsideEdge_1, insideEdge0_2}, {outsideEdge_1, insideEdge0_2}

Though it could probably be a little more helpful.

[disnet]

Agreed, in general the ambiguous subsets error needs to be more descriptive (say what and where the binding was) and this case in particular the error should mention re-declaration.