src/go/types/testdata/examples/constraints.go2

// Copyright 2021 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// This file shows some examples of generic constraint interfaces.

package p

type (
	// Type lists are processed as unions but an error is reported.
	// TODO(gri) remove this once the parser doesn't accept type lists anymore.
	_ interface{
		type /* ERROR use generalized embedding syntax instead of a type list */ int
	}
	_ interface{
		type /* ERROR use generalized embedding syntax instead of a type list */ int
		type float32
	}
)

type (
	// Arbitrary types may be embedded like interfaces.
	_ interface{int}
	_ interface{~int}

	// Types may be combined into a union.
	_ interface{int|~string}

	// Union terms must be unique independent of whether they are ~ or not.
	_ interface{int|int /* ERROR duplicate term int */ }
	_ interface{int|~ /* ERROR duplicate term int */ int }
	_ interface{~int|~ /* ERROR duplicate term int */ int }

	// For now we do not permit interfaces with methods in unions.
	_ interface{~ /* ERROR invalid use of ~ */ interface{}}
	_ interface{int|interface /* ERROR cannot use .* in union */ { m() }}
)

type (
	// Tilde is not permitted on defined types or interfaces.
	foo int
	bar interface{}
	_ interface{foo}
	_ interface{~ /* ERROR invalid use of ~ */ foo }
	_ interface{~ /* ERROR invalid use of ~ */ bar }
)

// Multiple embedded union elements are intersected. The order in which they
// appear in the interface doesn't matter since intersection is a symmetric
// operation.

type myInt1 int
type myInt2 int

func _[T interface{ myInt1|myInt2; ~int }]() T { return T(0) }
func _[T interface{ ~int; myInt1|myInt2 }]() T { return T(0) }

// Here the intersections are empty - there's no type that's in the type set of T.
func _[T interface{ myInt1|myInt2; int }]() T { return T(0 /* ERROR cannot convert */ ) }
func _[T interface{ int; myInt1|myInt2 }]() T { return T(0 /* ERROR cannot convert */ ) }