cmd/go2go: making a slice of generics struct doesn't compile #39936

sltc-li · 2020-06-30T02:43:22Z

What did you do?

https://go2goplay.golang.org/p/ZBpMABeQ0r8

package main

import (
	"fmt"
)

type Item(type T) struct {
	Val T
}

func NewItems(type T)(size int) []Item(T) {
	// Not compile: cannot use generic type Item(type T) without instantiation
	return make([]Item(T), size)
}

func main() {
	items := NewItems(int)(2)
	fmt.Printf("items = %#v\n", items)
}

What did you expect to see?

Making a slice of generics struct compiles

What did you see instead?

type checking failed

sltc-li · 2020-06-30T02:47:34Z

If I pre-define a generics type of the slice, it compiles.
https://go2goplay.golang.org/p/Wun2YoiGVcg

package main

import (
	"fmt"
)

type Item(type T) struct {
	Val T
}

type Items(type T) []Item(T)

func NewItems(type T)(size int) []Item(T) {
	return make(Items(T), size)
}

func main() {
	items := NewItems(int)(2)
	fmt.Printf("items = %#v\n", items)
}

tdakkota · 2020-06-30T03:40:38Z

If you add parentheses, it compiles too.
https://go2goplay.golang.org/p/OSGzqlKrMsM

return make([](Item(T)), size)

ianlancetaylor · 2020-06-30T03:43:15Z

This is another instance of the ambiguity described at https://go.googlesource.com/proposal/+/refs/heads/master/design/go2draft-type-parameters.md#instantiating-types-in-type-literals .

ianlancetaylor closed this as completed Jun 30, 2020

golang locked and limited conversation to collaborators Jun 30, 2021

gopherbot added the FrozenDueToAge label Jun 30, 2021

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cmd/go2go: making a slice of generics struct doesn't compile #39936

cmd/go2go: making a slice of generics struct doesn't compile #39936

sltc-li commented Jun 30, 2020

sltc-li commented Jun 30, 2020

tdakkota commented Jun 30, 2020

ianlancetaylor commented Jun 30, 2020

cmd/go2go: making a slice of generics struct doesn't compile #39936

cmd/go2go: making a slice of generics struct doesn't compile #39936

Comments

sltc-li commented Jun 30, 2020

What did you do?

What did you expect to see?

What did you see instead?

sltc-li commented Jun 30, 2020

tdakkota commented Jun 30, 2020

ianlancetaylor commented Jun 30, 2020