TH: panic in reportWarning when the message throws an exception

reportWarning and reportError doesn't force it's argument, so ghc panics when printing the message.

To reproduce:

test :: Q [Dec]
test = do
  reportWarning undefined
  return []

Output:

main.hs:5:3: Warning:ghc: panic! (the 'impossible' happened)
  (GHC version 7.8.1 for x86_64-unknown-linux):
	Prelude.undefined

Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

The fix could be:

--- a/compiler/typecheck/TcSplice.lhs
+++ b/compiler/typecheck/TcSplice.lhs
@@ -875,8 +875,8 @@ instance TH.Quasi (IOEnv (Env TcGblEnv TcLclEnv)) where
                   ; let i = getKey u
                   ; return (TH.mkNameU s i) }
 
-  qReport True msg  = addErr  (text msg)
-  qReport False msg = addWarn (text msg)
+  qReport True !msg  = addErr  (text msg)
+  qReport False !msg = addWarn (text msg)
 
   qLocation = do { m <- getModule
                  ; l <- getSrcSpanM

But possibly it should deepseq the argument?

Trac metadata

Trac field	Value
Version	7.8.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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