Impls for a generic type with different type parameters conflict with each other.

[vadimcn]

struct Foo<T>(T);
struct A;
struct B;

impl Foo<A> {
    fn foo() {}
}
impl Foo<B> {
    fn foo() {}
}

fn main() {}

produces duplicate definition error for foo:

test.rs:9:2: 9:13 error: duplicate definition of value `foo`
test.rs:9       fn foo() {}
                ^~~~~~~~~~~
test.rs:6:2: 6:13 note: first definition of value `foo` here
test.rs:6       fn foo() {}
                ^~~~~~~~~~~
error: aborting due to previous error

Since Foo<A> and Foo<B> are different types, it seems that their impls should not be interfering with each other.

[japaric]

Duplicate of #19097