What did the court decide in Conley v. Gibson?
Answered for: US
a complaint should not be dismissed for failure to state a claim unless it appears beyond doubt that the plaintiff can prove no set of facts in support of his claim which would entitle him to relief
The origin of the 'no set of facts' pleading formulation; the Supreme Court retired that phrase fifty years later in Bell Atlantic Corp. v. Twombly. The sentence above is the court in its own words.
This answer is true in US and nowhere else on this site. Another jurisdiction is another answer, or an honest gap: see what has been read for US.