Looks to me like it's confused about whether a ~ is part of an equality constraint or is a laziness annotation. The former would be illegal at that point, though, I'd think? Somewhere it believes a constraint might be possible there, via btype.

As ezyang says in the message I see just came in, you'll need extra production rules to distinguish that top level. Although I'd wonder why it believes an equality constraint is acceptable there in the first place; is that a lurking bug in the parser?
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