Python, checked before it runs.

$ tyc init hello && cd hello
Initialised Typhon project `hello` in ~/hello

$ tyc check src/
── errors in src/main.ty ──
tyc::nullable_use

  × possibly-None value used where `str` is required
    ╭─[src/main.ty:11:11]
 10 │     let user: str? = find_user(1)
 11 │     greet(user)
    ·           ──┬─
    ·             ╰── value is `str | None` here
    ╰────
  help: guard the value with `if user is not None:` to narrow it to `str`

# add the guard, then check again
$ tyc check src/
checked 1 file(s) — no errors
$ tyc run
hello, ada