Missing parentheses in closure signature produces bad diagnostic

[bnbarham]

Issue Kind

Bad Diagnostic Produced

Source Code

_ = { a: Int, b: Int in
}

Description

The diagnostic produced here is:

error: unexpected code ': Int, b: Int in' in closure

But it would be nice to instead have an error about missing parentheses (and a fix-it to add them). Apparently we're also not recovering to the in here properly, which isn't great.

rdar://112407661