Missing type parameter info when return type inferred from interface

[kLabz]

Hxb makes the issue visible when compiling this through compilation server:

interface Foo {
    function foo<T>():T;
    function bar<T>():T;
    function baz<T>():T;
}

class Bar implements Foo {
    public function foo() return null; // Warning: Unbound type parameter foo.T
    public function bar<T>() return null; // Error: write_full_path hxb writer failure
    public function baz<T>():T return null; // this is fine
}

While looking at what's happening (and so far failing addressing this issue...), I saw that cf_type was TFun([], TMono (Some (TAbstract(Null, [TMono (Some (TInst(T, [])))])))), with that T not having cl_kind = KTypeParameter ttp.

[Simn]

This is very much related to #11411. At this point I'm convinced that valid_redefinition needs a complete rewrite, but doing so requires a very particular state of being.