# D020 precondition_violated

URL: /docs/errors/D020
Markdown: /docs/errors/D020.md

> A requires clause failed on entry.

A runtime defect: the running program stops with it; no compile phase reports it, and `try` cannot catch it. `sakura explain D020` prints this explanation offline.

A `requires` clause failed on entry. The caller is blamed: the span is the call site. Check
the condition before the call, or give the argument a refined type whose predicate implies the
clause so the check happens where the value is made.

Runtime defects come from running programs, so `compiler/tests/ui` has no fixture for them; the explanation above names the repair.