isInstance does not work for compound types

Consider

import Language.Haskell.TH.Syntax                                                
                                                                                 
data A = A                                                                       
                                                                                 
test :: Q [Dec]                                                                  
test = do                                                                        
  tp <- [t| (Int, A) |]                                                          
  inst1 <- isInstance ''Show [ tp ]                                              
  runIO $ print inst1                                                            
  return []  

This prints True even though there is no Show instance for A (the Show instance it finds is for pairs, but it does not verify that there are Show instances for the pair components).

Trac metadata

Trac field	Value
Version	7.4.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Template Haskell
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