Can you please try with the latest Go master version? This bug might have been fixed in the week since beta2 came out. Note that you'll need to change the import of the constraints package as per #50792.

@mvdan Thanks for your reply quickly. I used gotip to get the master branch version:

go version devel go1.18-867a3d5 Mon Feb 7 12:32:51 2022 +0000 darwin/arm64

and replaced constraints with golang.org/x/exp/constraints to import it, as the method in that issue. But unfortunately, the problem still exists

Ah, could you please push to your repo so we can also reproduce with master?

Okay, it's there now

The problem is fixed if you change

func Sum[T []E, E constraints.Integer | constraints.Float](items T) (total E) {

to

func Sum[T interface{ []E }, E constraints.Integer | constraints.Float](items T) (total E) {

or

func Sum[T ~[]E, E constraints.Integer | constraints.Float](items T) (total E) {

As the error message indicates, []E is not an interface constraint. You can wrap it in interface{} to make it an interface constraint. But we also consider a type U to be equivalent to interface { U } (a real type constraint) if U contains either a tilde or a bar.

@griesemer @findleyr

Robert/Rob: Things do in fact work if foo.go is renamed to hello.go. Also, we are not complaining about the use of T []E in MakeCollection. So, the error message about []E not be an interface (hence not a proper constraint) seems to not be provided consistently in all cases. Would be good to investigate why it is not happen when foo.go is renamed to hello.go, why it's not happening for MakeCollection, etc.

It looks like an ordering problem (hence the dependence of the filename, the sources are processed by the type checker in filename-sorted order for deterministic behavior).

Simpler reproducer:

package p

// func f[T int]() {}  // using this f doesn't produce the error

func _[T int]() {
	_ = f[T] // ERROR T does not implement int (int is not an interface)
}

func f[T int]() {}  // using this f produces the error

Change https://go.dev/cl/383834 mentions this issue: go/types, types2: ensure we have an interface before checking constraints

@griesemer Thanks for your work. Is complaining about mistakes to be expected? Because I find it works well everywhere else except complaining

@sxyazi I'm not sure I understand: what do you mean with "except complaining"? Do you mean that compiler is complaining with errors?

With respect to this issue: this was a real bug in the compiler; thanks for bringing it up to our attention. I believe it should be fixed with the above CL. (And yes, if you find more cases that you think should work but don't, please file an issue.)

@griesemer OK, I see. Thanks for your work again.