ImplicitParams: GHC does not warn about unused implicit parameters

When using -XImplicitParams, and an implicit parameter is not used within a function which accepts said parameter, no warning is issued (with -Wall turned on).

Example:

{-# LANGUAGE ImplicitParams #-}

f :: (?file1 :: String) => IO ()
f = putStrLn $ "f2: "

main :: IO ()
main = let ?file1 = "A" in f

Here, I would expect a warning about ?file1 being unused in f.

As far as I can tell (but I'm quite the layman in these matters), issuing a warning here would be desirable, possible, and quite easy. Sorry for the bug report if there's a reason for the status quo.

Trac metadata

Trac field	Value
Version	7.10.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	lowest
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture

changed weight to 0

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I think it works fine in HEAD. Can others verify?

T10632.hs:5:6: warning:
    Redundant constraint: ?file1::String
    In the type signature for: f :: (?file1::String) => IO ()

If so, can someone add this as a regression test?

I don't think it's worth messing with 7.10.

Simon

mentioned in commit 9834fea4

closed

Trac metadata

Trac field	Value
Resolution	Unresolved → ResolvedFixed

changed milestone to %8.0.1

mentioned in issue #11370 (closed)

We may need to disable these warnings by default (including with -Wall). See #11370 (closed) for details.

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