Better treatment for dataToTag

In libraries/Cabal/Cabal/Distribution/System.hs I found

eqOS :: OS -> OS -> Bool
eqOS (OtherOS s1) (OtherOS s2) = s1 == (s2 :: String)
eqOs a b = dataToTag a == dataTag b

(actually it's not called eqOS; it's the derived equality for OS).

By the time this gets to Core it looks something like this

eqOS a b
  = case a of
      OtherOS s1 -> case b of
                       OtherOS s2 -> eqString s1 s2
                       _ -> case dataToTag b of
                              16# -> True
                              r   -> False
      _ -> dataToTag a == dataToTag b

The dataToTag code has been duplicated; and in the OtherOS s1 branch GHC can constant-fold the dataToTag on a to 16#, the tag of OtherOS. But GHC is no so clever for the dataToTag b. We know that b is not OtherOS, so we know its tag is not 16#, so the innermost case is entirely redundant. But we don't spot that.

Trac metadata

Trac field	Value
Version	8.2.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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