src/internal/types/testdata/spec/conversions.go

// 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.

package conversions

import "unsafe"

// constant conversions

func _[T ~byte]() T { return 255 }
func _[T ~byte]() T { return 256 /* ERRORx `cannot use 256 .* as T value` */ }

func _[T ~byte]() {
	const _ = T /* ERRORx `T\(0\) .* is not constant` */ (0)
	var _ T = 255
	var _ T = 256 // ERRORx `cannot use 256 .* as T value`
}

func _[T ~string]() T                { return T('a') }
func _[T ~int | ~string]() T         { return T('a') }
func _[T ~byte | ~int | ~string]() T { return T(256 /* ERRORx `cannot convert 256 .* to type T` */) }

// implicit conversions never convert to string
func _[T ~string]() {
	var _ string = 0 // ERRORx `cannot use .* as string value`
	var _ T = 0      // ERRORx `cannot use .* as T value`
}

// failing const conversions of constants to type parameters report a cause
func _[
	T1 any,
	T2 interface{ m() },
	T3 ~int | ~float64 | ~bool,
	T4 ~int | ~string,
]() {
	_ = T1(0 /* ERRORx `cannot convert 0 .* to type T1: T1 does not contain specific types` */)
	_ = T2(1 /* ERRORx `cannot convert 1 .* to type T2: T2 does not contain specific types` */)
	_ = T3(2 /* ERRORx `cannot convert 2 .* to type T3: cannot convert 2 .* to type bool \(in T3\)` */)
	_ = T4(3.14 /* ERRORx `cannot convert 3.14 .* to type T4: cannot convert 3.14 .* to type int \(in T4\)` */)
}

// "x is assignable to T"
// - tested via assignability tests

// "x's type and T have identical underlying types if tags are ignored"

func _[X ~int, T ~int](x X) T { return T(x) }
func _[X struct {
	f int "foo"
}, T struct {
	f int "bar"
}](x X) T {
	return T(x)
}

type Foo struct {
	f int "foo"
}
type Bar struct {
	f int "bar"
}
type Far struct{ f float64 }

func _[X Foo, T Bar](x X) T       { return T(x) }
func _[X Foo | Bar, T Bar](x X) T { return T(x) }
func _[X Foo, T Foo | Bar](x X) T { return T(x) }
func _[X Foo, T Far](x X) T {
	return T(x /* ERROR "cannot convert x (variable of type X constrained by Foo) to type T: cannot convert Foo (in X) to type Far (in T)" */)
}

// "x's type and T are unnamed pointer types and their pointer base types
// have identical underlying types if tags are ignored"

func _[X ~*Foo, T ~*Bar](x X) T         { return T(x) }
func _[X ~*Foo | ~*Bar, T ~*Bar](x X) T { return T(x) }
func _[X ~*Foo, T ~*Foo | ~*Bar](x X) T { return T(x) }
func _[X ~*Foo, T ~*Far](x X) T {
	return T(x /* ERROR "cannot convert x (variable of type X constrained by ~*Foo) to type T: cannot convert *Foo (in X) to type *Far (in T)" */)
}

// Verify that the defined types in constraints are considered for the rule above.

type (
	B  int
	C  int
	X0 *B
	T0 *C
)

func _(x X0) T0           { return T0(x /* ERROR "cannot convert" */) } // non-generic reference
func _[X X0, T T0](x X) T { return T(x /* ERROR "cannot convert" */) }
func _[T T0](x X0) T      { return T(x /* ERROR "cannot convert" */) }
func _[X X0](x X) T0      { return T0(x /* ERROR "cannot convert" */) }

// "x's type and T are both integer or floating point types"

func _[X Integer, T Integer](x X) T  { return T(x) }
func _[X Unsigned, T Integer](x X) T { return T(x) }
func _[X Float, T Integer](x X) T    { return T(x) }

func _[X Integer, T Unsigned](x X) T  { return T(x) }
func _[X Unsigned, T Unsigned](x X) T { return T(x) }
func _[X Float, T Unsigned](x X) T    { return T(x) }

func _[X Integer, T Float](x X) T  { return T(x) }
func _[X Unsigned, T Float](x X) T { return T(x) }
func _[X Float, T Float](x X) T    { return T(x) }

func _[X, T Integer | Unsigned | Float](x X) T { return T(x) }
func _[X, T Integer | ~string](x X) T {
	return T(x /* ERROR "cannot convert x (variable of type X constrained by Integer | ~string) to type T: cannot convert string (in X) to type int (in T)" */)
}

// "x's type and T are both complex types"

func _[X, T Complex](x X) T { return T(x) }
func _[X, T Float | Complex](x X) T {
	return T(x /* ERROR "cannot convert x (variable of type X constrained by Float | Complex) to type T: cannot convert float32 (in X) to type complex64 (in T)" */)
}

// "x is an integer or a slice of bytes or runes and T is a string type"

type myInt int
type myString string

func _[T ~string](x int) T      { return T(x) }
func _[T ~string](x myInt) T    { return T(x) }
func _[X Integer](x X) string   { return string(x) }
func _[X Integer](x X) myString { return myString(x) }
func _[X Integer](x X) *string {
	return (*string)(x /* ERROR "cannot convert x (variable of type X constrained by Integer) to type *string: cannot convert int (in X) to type *string" */)
}

func _[T ~string](x []byte) T                           { return T(x) }
func _[T ~string](x []rune) T                           { return T(x) }
func _[X ~[]byte, T ~string](x X) T                     { return T(x) }
func _[X ~[]rune, T ~string](x X) T                     { return T(x) }
func _[X Integer | ~[]byte | ~[]rune, T ~string](x X) T { return T(x) }
func _[X Integer | ~[]byte | ~[]rune, T ~*string](x X) T {
	return T(x /* ERROR "cannot convert x (variable of type X constrained by Integer | ~[]byte | ~[]rune) to type T: cannot convert int (in X) to type *string (in T)" */)
}

// "x is a string and T is a slice of bytes or runes"

func _[T ~[]byte](x string) T { return T(x) }
func _[T ~[]rune](x string) T { return T(x) }
func _[T ~[]rune](x *string) T {
	return T(x /* ERROR "cannot convert x (variable of type *string) to type T: cannot convert *string to type []rune (in T)" */)
}

func _[X ~string, T ~[]byte](x X) T           { return T(x) }
func _[X ~string, T ~[]rune](x X) T           { return T(x) }
func _[X ~string, T ~[]byte | ~[]rune](x X) T { return T(x) }
func _[X ~*string, T ~[]byte | ~[]rune](x X) T {
	return T(x /* ERROR "cannot convert x (variable of type X constrained by ~*string) to type T: cannot convert *string (in X) to type []byte (in T)" */)
}

// package unsafe:
// "any pointer or value of underlying type uintptr can be converted into a unsafe.Pointer"

type myUintptr uintptr

func _[X ~uintptr](x X) unsafe.Pointer  { return unsafe.Pointer(x) }
func _[T unsafe.Pointer](x myUintptr) T { return T(x) }
func _[T unsafe.Pointer](x int64) T {
	return T(x /* ERROR "cannot convert x (variable of type int64) to type T: cannot convert int64 to type unsafe.Pointer (in T)" */)
}

// "and vice versa"

func _[T ~uintptr](x unsafe.Pointer) T  { return T(x) }
func _[X unsafe.Pointer](x X) uintptr   { return uintptr(x) }
func _[X unsafe.Pointer](x X) myUintptr { return myUintptr(x) }
func _[X unsafe.Pointer](x X) int64 {
	return int64(x /* ERROR "cannot convert x (variable of type X constrained by unsafe.Pointer) to type int64: cannot convert unsafe.Pointer (in X) to type int64" */)
}

// "x is a slice, T is an array or pointer-to-array type,
// and the slice and array types have identical element types."

func _[X ~[]E, T ~[10]E, E any](x X) T  { return T(x) }
func _[X ~[]E, T ~*[10]E, E any](x X) T { return T(x) }

// ----------------------------------------------------------------------------
// The following declarations can be replaced by the exported types of the
// constraints package once all builders support importing interfaces with
// type constraints.

type Signed interface {
	~int | ~int8 | ~int16 | ~int32 | ~int64
}

type Unsigned interface {
	~uint | ~uint8 | ~uint16 | ~uint32 | ~uint64 | ~uintptr
}

type Integer interface {
	Signed | Unsigned
}

type Float interface {
	~float32 | ~float64
}

type Complex interface {
	~complex64 | ~complex128
}