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// check-pass struct D; trait Tr { type It; fn foo(self) -> Option<Self::It>; } impl<'a> Tr for &'a D { type It = (); fn foo(self) -> Option<()> { None } } fn run<F>(f: F) where for<'a> &'a D: Tr, F: Fn(<&D as Tr>::It), { let d = &D; while let Some(i) = d.foo() { f(i); } } fn main() { run(|_| {}); }