Type spreads of regular variants in patterns #6721
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Solves part of #6273
Closes #6562
Closes #6277
This PoC adds support for type spreads of regular variants in patterns. This is already a thing with polyvariants, and now it's also a thing for regular variants.
Rationale is we already have variant spreads and coercion, which lets you go from small type to big type. Having this will let you go from big type to small type. This is a missing piece.
Specification:
X
that is a subtype of variantY
can be spread in a pattern matching of a value of typeY
...subtypeVariant as value
will give the typesubTypeVariant
tovalue
#
:| ...subtypeVariant as value => doSomethingWithSubTypeVariant(value)
Should be a well isolated feature, since it's dependent on the parser adding an attribute when the spread is a regular variant (no leading
#
) and no regular pre-existing code path is touched.