"tcfail023.hs", line 8: Duplicate/overlapping instances:
    class "A", type "B"; at "tcfail023.hs", line 8 and "tcfail023.hs", line 11
 
"tcfail023.hs", line 11:
    Couldn't match type "Bool" against "B".
    Inside a function binding:
	op ... C = True
Fail: Compilation errors found

ghc: execution of the Haskell compiler had trouble