type MyInt int

func foo2[T int]() T {
	x := 42
	return int(x) // ERROR: cannot use int(x) (value of type int) as type T in return statement
}

func foo2[T ~int]() T {
	x := 42
	return T(int(x)) // OK
}

package main

type A struct{}

func (a A) Foo() {}

type B struct{}

func (b B) Foo() {}

type C interface{ Foo() }

func Want[T C]() *T {

	var x T
	switch (interface{})(x).(type) {
	case A:
		return (*T)(&A{}) // cannot convert &A{} (value of type *A) to type *T: *A does not implement *T (type *T is pointer to interface, not interface)
	case B:
		return (*T)(&B{}) // cannot convert &B{} (value of type *B) to type *T: *B does not implement *T (type *T is pointer to interface, not interface)
	default:
		panic("unknown")
	}

}

func main() {
	Want[A]()
	Want[B]()
}

package main

import "unsafe"

type A struct{}

func (a A) Foo() {}

type B struct{}

func (b B) Foo() {}

type C interface{ Foo() }

func Want[T C]() *T {

	var x T
	switch (interface{})(x).(type) {
	case A:
		return (*T)(unsafe.Pointer(&A{})) // OK
	case B:
		return (*T)(unsafe.Pointer(&B{})) // OK
	default:
		panic("unknown")
	}

}

func main() {
	Want[A]()
	Want[B]()
}