cmd/compile: cannot return a value even its type subjects to type constraints

[changkun]

Does this issue reproduce with the latest release?

Yes

What did you do?

https://go.dev/play/p/pCzjLWAepvO?v=gotip

package main

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

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 main() {}

What did you expect to see?

Compile pass.

What did you see instead?

Compile error.

[zigo101]

type MyInt int

MyInt might be used as an argument passed to foo2 type parameter. An int value may not be assigned to a MyInt value.

In fact, even if the foo2 function is change to

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
}

it still doesn't compile. Please view #51403 for details.

[changkun]

Sigh, sounds very limited. Though I noticed that we could:

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

But use cases like this still can't work:

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]()
}

One have to import unsafe:

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]()
}

[zigo101]

The pointer conversion problem is the same as: #50815

The key point here is the underlying type of T is an interface type. In fact, the underlying type of every type parameter type is an interface type.