weaken_result_type: capture the previous type in the wp, but only if the computation is impure and previous type is not same as the type we are trying to weaken it to (cf. #1470, #1500)
weaken_result_type: capture the previous type in the wp, but only if the computation is impure and previous type is not same as the type we are trying to weaken it to (cf. #1470, #1500)

FStarLang/FStar,FStarLang/FStar,A-Manning/FStar,A-Manning/FStar,FStarLang/FStar,A-Manning/FStar