Program doesn't preserve semantics after pattern synonym inlining.

{-# language
   PatternSynonyms
 , ViewPatterns
 #-}

import Text.Read

-- pattern PRead :: () => Read a => a -> String
pattern PRead a <- (readMaybe -> Just a)

foo :: String -> Int
foo (PRead x)  = (x::Int)
foo (PRead xs) = sum (xs::[Int])
foo _ = 666

bar :: String -> Int
bar (readMaybe -> Just x)  = (x::Int)
bar (readMaybe -> Just xs) = sum (xs::[Int])
bar _ = 666

main :: IO ()
main = do
  print $ foo "1"       -- 1
  print $ foo "[1,2,3]" -- 666 -- ???
  print $ foo "xxx"     -- 666

  print $ bar "1"       -- 1
  print $ bar "[1,2,3]" -- 6
  print $ bar "xxx"     -- 666

I'm expecting that semantics of the function foo would not change if I inline pattern synonym.

However it does. bar successfully matches string against list of ints, whereas foo apperently skips this pattern and fall back to the catch-all case.

Reproducible both in GHCi and GHC.

Trac metadata

Trac field	Value
Version	7.10.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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