Nested generic impl

Question

I don't understand why rust don't allow to use a generic inside an other generic constraint.

It's difficult to explain with normal words but for example this simple code don't work

trait GenericTraitA<T1> {}

trait GenericTraitB<T2> {}

impl<T1, T2: GenericTraitA<T1>> dyn GenericTraitB<T2> {}

It says that:

the type parameter T1 is not constrained by the impl trait, self type, or predicates

But i don't see why this is not constrained, i don't understand the ambiguity inside this code.