UB cannot be prevented in this case:   Not all pre-conditions can be either easily specified or easily proven by the compiler.   Not following implicit pre-conditions may lead to UB.   The UB may already have been compiled into a third-party dynamic or static library.   Even informing the compiler at the call site about why the UB never happens, would not work for implicit pre-conditions and separately compiled UB.      -----Ursprüngliche Nachricht----- Von:Peter Bindels via Std-Proposals <std-proposals_at_[hidden]> Gesendet:Mi 19.08.2026 15:31 Betreff:Re: [std-proposals] P4334R0 C++ Contracts An:std-proposals_at_[hidden]; CC:Peter Bindels <dascandy_at_[hidden]>;   On Wed, Aug 19, 2026 at 3:26 PM Rainer Deyke via Std-Proposals <std-proposals_at_[hidden] <mailto:std-proposals_at_[hidden]> > wrote: On 8/19/26 05:37, Ville Voutilainen via Std-Proposals wrote: > On Wed, 19 Aug 2026 at 05:54, Sebastian Wittmeier via Std-Proposals > <std-proposals_at_[hidden] <mailto:std-proposals_at_[hidden]> > wrote: >> >> If P2900 is staying, can you define (= propose) a 5th mode that acts like compile_assert and also hope that the implementations give better error messages and locations than an undefined violation handler? > > I don't know about others, but yes, yes I can: https://godbolt.org/z/xPz8hjcYY I don't see the precondition violation.  `use_definitely_unsound` is a perfectly fine function for getting a -1 so long as its implicit precondition of not passing in a vector of exactly 5 elements is met. Nothing in the program violates this implicit precondition. Implicit preconditions still exist, right?  There is no way to encode every precondition into something the compiler understands.  If a branch exists with definite UB but it's never called is the program still ill-formed? For my own software I want that to be yes, but for C++ in general I think so far we have no, and changing such a thing will be a breaking change for many. -- Std-Proposals mailing list Std-Proposals_at_[hidden] https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals