Union doesn't support generics in the last version

[vpikulik]

Expected Behavior

Check this example master...vpikulik:union_fail_example
It compiled in previous versions

Actual Behavior

$ -> cargo check
    Checking example-openapi-union v0.1.0 (/_____/poem/examples/openapi/union)
error[E0401]: can't use generic parameters from outer item
  --> openapi/union/src/main.rs:18:9
   |
17 | enum MyObj<T: Type + ParseFromJSON + ToJSON> {
   |            - type parameter from outer item
18 |     A(A<T>),
   |         ^ use of generic parameter from outer item

For more information about this error, try `rustc --explain E0401`.
error: could not compile `example-openapi-union` (bin "example-openapi-union") due to 1 previous error

Steps to Reproduce the Problem

1. switch to the branch https://github.com/vpikulik/poem/tree/union_fail_example
2. cd poem/examples/openapi/union
3. cargo check

Specifications

• Version: rustc 1.77.2
• Platform:
• Subsystem:

The reason of the problem

cargo expand generates next code:

                    const _: fn() = || {
                        fn assert_impl_all<
                            T: ?Sized + poem_openapi::types::IsObjectType,
                        >() {}
                        assert_impl_all::<A<T>>();
                    };

I think crate static_assertions should be replaced.

[wpikulik]

It looks like the issue was introduced in this commit 869d7c0

The issue #800 also caused by this change.