Union pattern match failure

[ldcouto]

Minimal example:

p let ({} union {}) union {} = {} in 1
= 1

See overturetool/documentation#9 for more

[nickbattle]

The problem is caused by the fact that the brackets make the LHS into an expression pattern, and these were always considered to be "length 1" (ie. yielding a set with one member in this context). In fact an expression can clearly result in an arbitrary length, including zero. So the length for an expression pattern should be ANY.

[nickbattle]

Now available in ncb/development.