cmd/compile: better error message for type arguments used with unsatisfiable constraints (empty type sets)

[komuw]

What did you do?

Ran this program: https://go2goplay.golang.org/p/S4MLyTi6yA-

package main

import (
	"fmt"
)

type interger32 interface {
	type int32
}

type number interface {
	type float64, int, int32
	interger32
}

func Largest[type T number](s []T) T {
	var largest T
	for _, v := range s {
		if v > largest {
			largest = v
		}
	}
	return largest
}

func main() {
	arr := []int{101, -1, 45, 646, 3}
	l := Largest[int](arr)

	fmt.Printf("largest in %v is: %v", arr, l)
}

What did you expect to see?

• Either it runs or produces a comprehensible error message about int32 been repeated in interfaces interger32 & number

What did you see instead?

type checking failed for main
prog.go2:28:15: int does not satisfy number (int not found in int32)

[griesemer]

Just for the record, the problem is not that int32 is repeated in both interfaces, the problem is that int is not repeated in both interfaces: When interfaces are embedded, all the type lists are intersected. This way we can guarantee that if a type T satisfies an interface that embeds an interface E, T also satisfies E.

Agreed that we could use a better error message. Not urgent.

[komuw]

seems resolved at tip.

[griesemer]

I'm going to re-open this. The error message at tip could be better. For:

package p

type interger32 interface {
	~int32
}

type number interface {
	~float64 | ~int | ~int32
	interger32
}

func f[T number]() {}

func _() {
	_ = f[int]
}

we get:

x.go:15:8: int does not satisfy number

We should say a bit more as to why.

[gopherbot]

Change https://go.dev/cl/410894 mentions this issue: go/types, types2: use | rather than ∪ when printing term lists

[gopherbot]

Change https://go.dev/cl/410876 mentions this issue: go/types, types2: better error message if type is not in type set