Spurious "defined but not used" warnings for constructors of types deriving Read

Consider the following code:

  data D = D { ds :: String, db :: Bool }
    deriving (Read, Show)

  main :: IO ()
  main = do
    let s = "D { ds = \"oi\", db = True }"
    print (ds (read s))
    print (db (read s))

With -Wall, ghc emits the following warning:

  Warning: Defined but not used: data constructor `D'

I think this warning is not justified, because the D constructor is used to parse s.

The testcase above was derived from a program where D held a bunch of configuration fields for the program, and s was read from configuration file.

I would think the fix should be a rule along the lines of: if a data type derives from Read, then all of its constructors and field names are considered used.

Trac metadata

Trac field	Value
Version	6.8.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	low
Resolution	Unresolved
Component	Compiler
Test case
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Operating system	Unknown
Architecture	Unknown