Type checking arrow notation in the presence of deferred constraints

In TcArrows, the following code

    tc_cmd w_tv (cmd, i, b, tup_ty, s)
      = do { tup_ty' <- zonkTcType tup_ty
	   ; let (corner_ty, arg_tys) = unscramble tup_ty'

		-- Check that it has the right shape:
		-- 	((w,s1) .. sn)
		-- where the si do not mention w
	   ; checkTc (corner_ty `eqType` mkTyVarTy w_tv && 
		      not (w_tv `elemVarSet` tyVarsOfTypes arg_tys))
		     (badFormFun i tup_ty')

	   ; tcCmdTop (env { cmd_arr = b }) cmd arg_tys s }

is wrong. In particular, the check elemVarSet w_tv (tyVarsOfTypes arg_tys) does not make sense in the presence of deferred constraints.

There is no testcase for this at the moment, but in the ghc-kinds branch both T5283 and arrowform1 fail because of this. The temporary fix was:

           ; _bogus <- unifyType corner_ty (mkTyVarTy w_tv)
	   ; checkTc (not (w_tv `elemVarSet` tyVarsOfTypes arg_tys))
		     (badFormFun i tup_ty')

This solves only T5283 and arrowform1; other problems might arise in the future, so this should be fixed properly.

Trac metadata

Trac field	Value
Version	7.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
Differential revisions
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Architecture