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Created by Andreas Abel@abel

Unsound optimization with `error`, introduced in GHC 8.2

We discovered an unsound optimization (see https://github.com/agda/agda-stdlib/pull/1512) in GHC starting with 8.2. I managed to shrink it down to the following code:

-- Unsound compiler optimization in presence of @error@.

foo :: String -> String
foo d1@(_:_)
  | not (all (== '-') d1)
  , last d1 == '\r'
  = error "OK1"
foo _ = error "OK2"

main = putStr $ foo "Hello, world!"

With -O0, or until GHC 8.0.2, this outputs

    <main>: OK2
    CallStack (from HasCallStack):
      error, called at <loc> in main:Main

With -O1 or -O2 and GHC 8.2 - 9.0.1, (I tested from 8.2.2), this gives:

    Prelude.last: empty list