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Summary

{-# OPTIONS_GHC -Wincomplete-record-updates #-}
data Foo = A {a :: Int} | B deriving Show

foo = A 4

-- wibble is safe - no warning
wibble = do
  case foo of
    bar@A{} -> Just bar{a = 9}
    _ -> fail ":("

-- using guards doesn't throw a warning
twomble | bar@A{} <- foo = Just bar{a = 9}
        | otherwise  = fail ":("

-- sworble has the same semantics as wibble and twomble - but we get a warning!
sworble = do
  bar@A{} <- Just foo
  Just bar{a = 9}

Output:

DoBug.hs:19:8: warning: [-Wincomplete-record-updates]
    Pattern match(es) are non-exhaustive
    In a record-update construct: Patterns of type `Foo' not matched: B
   |
19 |   Just bar{a = 9}
   |        ^^^^^^^^^^

Expected behavior

Don't warn

Environment

• GHC version used: 9.3.20220401