For the case of coercing a specifically a function argument to a different interface, there is a solution that isn't stupidly inefficient, but it's impossible to support for the general case of coercing any interface value to a different interface. I expect 90% of the time a function coerces and interface value to a different interface, that value is one of its arguments, but to support that case for generic interfaces but not the other 10% would be a really ugly design. I think you'd also have issues with treating functions that coerce their arguments to generic interfaces as first class functions.

    package p1
    type S struct{}
    func (S) Identity[T any](v T) T { return v }

    package p2
    type HasIdentity[T any] interface {
     Identity(T) T
    }

    package p3
    import "p2"
    // Because parameter v might be (and in this case is) coerced into
    // p2.HasIdentity[int], this function gets a type annotation in the compiler
    // output indicating that callers should pass a p2.HasIdentity[int] interface
    // reference for v if it exists, or nil if it doesn't, as an additional
    // argument.
    func CheckIdentity(v interface{}) {
     if vi, ok := v.(p2.HasIdentity[int]); ok {
      if got := vi.Identity(0); got != 0 {
       panic(got)
      }
     }
    }

    package p4
    import (
     "p1"
     "p3"
    )
    func CheckSIdentity() {
     p3.CheckIdentity(p1.S{})
    }