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cmd/go2go: inappropriate "cannot use generic type without instantiation" error #40791

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rogpeppe opened this issue Aug 14, 2020 · 3 comments
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cmd/go2go: inappropriate "cannot use generic type without instantiation" error #40791

rogpeppe opened this issue Aug 14, 2020 · 3 comments
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@rogpeppe
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rogpeppe commented Aug 14, 2020 •
edited

commit 722af87

The following program fails with the error "cannot use generic type Pair(type A, B) without instantiation", but the type does seem to be instantiated correctly.

package main
import "fmt"

func main() {
	fmt.Println(zeroS(int)())
}

type S(type T) struct {
	T T
}

func zeroS(type T)() S(T) {
	m := make([]S(T), 1)
	return m[0]
}

If we add a type alias, it works.
@ainar-g
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ainar-g commented Aug 14, 2020

I don't know if it's related, but it seems to work when you use square brackets.

@rogpeppe
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Ah, interesting! I was using round brackets because originally I had more than one type parameter so couldn't use square brackets because of issue #40788. https://go2goplay.golang.org/p/v35cQ-jcbEd

@ianlancetaylor
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Yes, this is the syntactic ambiguity of []S(T) when using parentheses. You have to write it as [](S(T)). Or use square brackets.

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@rogpeppe @ianlancetaylor @ainar-g @gopherbot