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A `where` clause contains a nested quantification over lifetimes.
Erroneous code example:
```compile_fail,E0316
trait Tr<'a, 'b> {}
fn foo<T>(t: T)
where
for<'a> &'a T: for<'b> Tr<'a, 'b>, // error: nested quantification
{
}
```
Rust syntax allows lifetime quantifications in two places within
`where` clauses: Quantifying over the trait bound only (as in
`Ty: for<'l> Trait<'l>`) and quantifying over the whole clause
(as in `for<'l> &'l Ty: Trait<'l>`). Using both in the same clause
leads to a nested lifetime quantification, which is not supported.
The following example compiles, because the clause with the nested
quantification has been rewritten to use only one `for<>`:
```
trait Tr<'a, 'b> {}
fn foo<T>(t: T)
where
for<'a, 'b> &'a T: Tr<'a, 'b>, // ok
{
}
```
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