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a91d0b649c
We may revisit this decision in a future release. By disallowing this for Go 1.18 we are ensuring that we don't lock in the generics design in a place that may need to change later. (Type declarations are the primary construct where it crucially matters what the underlying type of a type parameter is.) Comment out all tests that rely on this feature; add comments referring to issue so we can find all places easily should we change our minds. Fixes #45639. Change-Id: I730510e4da66d3716d455a9071c7778a1e4a1152 Reviewed-on: https://go-review.googlesource.com/c/go/+/359177 Trust: Robert Griesemer <gri@golang.org> Trust: Dan Scales <danscales@google.com> Reviewed-by: Dan Scales <danscales@google.com>
109 lines
2.7 KiB
Go
109 lines
2.7 KiB
Go
// run -gcflags=-G=3
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// Copyright 2021 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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// This test illustrates how a type bound method (String below) can be implemented
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// either by a concrete type (myint below) or a instantiated generic type
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// (StringInt[myint] below).
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package main
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import (
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"fmt"
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"reflect"
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"strconv"
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)
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type myint int
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//go:noinline
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func (m myint) String() string {
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return strconv.Itoa(int(m))
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}
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type Stringer interface {
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String() string
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}
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func stringify[T Stringer](s []T) (ret []string) {
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for _, v := range s {
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// Test normal bounds method call on type param
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x1 := v.String()
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// Test converting type param to its bound interface first
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v1 := Stringer(v)
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x2 := v1.String()
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// Test method expression with type param type
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f1 := T.String
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x3 := f1(v)
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// Test creating and calling closure equivalent to the method expression
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f2 := func(v1 T) string {
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return Stringer(v1).String()
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}
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x4 := f2(v)
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if x1 != x2 || x2 != x3 || x3 != x4 {
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panic(fmt.Sprintf("Mismatched values %v, %v, %v, %v\n", x1, x2, x3, x4))
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}
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ret = append(ret, v.String())
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}
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return ret
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}
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type Ints interface {
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~int32 | ~int
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}
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// For now, a lone type parameter is not permitted as RHS in a type declaration (issue #45639).
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// type StringInt[T Ints] T
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//
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// //go:noinline
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// func (m StringInt[T]) String() string {
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// return strconv.Itoa(int(m))
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// }
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type StringStruct[T Ints] struct {
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f T
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}
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func (m StringStruct[T]) String() string {
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return strconv.Itoa(int(m.f))
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}
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func main() {
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x := []myint{myint(1), myint(2), myint(3)}
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// stringify on a normal type, whose bound method is associated with the base type.
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got := stringify(x)
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want := []string{"1", "2", "3"}
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if !reflect.DeepEqual(got, want) {
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panic(fmt.Sprintf("got %s, want %s", got, want))
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}
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// For now, a lone type parameter is not permitted as RHS in a type declaration (issue #45639).
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// x2 := []StringInt[myint]{StringInt[myint](5), StringInt[myint](7), StringInt[myint](6)}
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//
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// // stringify on an instantiated type, whose bound method is associated with
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// // the generic type StringInt[T], which maps directly to T.
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// got2 := stringify(x2)
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// want2 := []string{"5", "7", "6"}
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// if !reflect.DeepEqual(got2, want2) {
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// panic(fmt.Sprintf("got %s, want %s", got2, want2))
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// }
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// stringify on an instantiated type, whose bound method is associated with
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// the generic type StringStruct[T], which maps to a struct containing T.
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x3 := []StringStruct[myint]{StringStruct[myint]{f: 11}, StringStruct[myint]{f: 10}, StringStruct[myint]{f: 9}}
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got3 := stringify(x3)
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want3 := []string{"11", "10", "9"}
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if !reflect.DeepEqual(got3, want3) {
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panic(fmt.Sprintf("got %s, want %s", got3, want3))
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}
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}
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