Existing blanket impl From<T> for T makes real uses for generic types impossible

[mqudsi]

The blanket impl From<T> for T definition in core makes actual impls of std::convert::From for generic types extremely difficult.

Consider:

enum Subtype {
    Type1,
    Type2
}

struct Foo<T> where T: Subtype {
// ...
}

It is natural to wish to implement conversion between the subtypes, and to want to do so with as little repetition as possible. But

impl<Src: Subtype, Dst: Subtype> From<Foo<Src>> for Foo<Dst> {
...
}

is not possible:

   Compiling chrono v0.4.5 (file:///mnt/d/rand/chrono)
error[E0119]: conflicting implementations of trait `std::convert::From<datetime::DateTime<_>>` for type `datetime::DateTime<_>`:
   --> src/datetime.rs:238:1
    |
238 | impl<Src: TimeZone, Dst: TimeZone> From<DateTime<Src>> for DateTime<Dst> {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: conflicting implementation in crate `core`:
            - impl<T> std::convert::From<T> for T;

error: aborting due to previous error

For more information about this error, try `rustc --explain E0119`.
error: Could not compile `chrono`.

To learn more, run the command again with --verbose.
Makefile:17: recipe for target 'build' failed
make: *** [build] Error 101

Could the generic impl be made more specific so that it accounts for the fact that Foo<T1> and Foo<T2> are not the same, to permit user-defined conversion between them?