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cmd/compile/internal/types2: review implementation of indexing of generic types #49275

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danscales opened this issue Nov 1, 2021 · 3 comments
Closed

cmd/compile/internal/types2: review implementation of indexing of generic types #49275

danscales opened this issue Nov 1, 2021 · 3 comments
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FrozenDueToAge NeedsFix The path to resolution is known, but the work has not been done. release-blocker
Milestone
Go1.18

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@danscales
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On go tip (generics), the below program gets a compile error

./test.go:6:15: invalid operation: cannot index x (variable of type T constrained by interface{[]int64|map[int]int64})

Program:

package main

import "fmt"

func g[T interface{ []int64 | map[int]int64 }](x T) int64 {
	return x[3])
}

func main() {
	x := make(map[int]int64)
	y := make([]int64, 4)

	fmt.Printf("%v %v\n", g(x), g(y))
}

That happens even though individually the functions:

func g[T interface{ []int64 }](x T) int64 {
	return x[3]
}

and

func g[T interface{ map[int]int64 }](x T) int64 {
	return x[3]
}

compile/run fine. We need a fix and/or need to decide exactly which of map/slice/array/string types can be combined in a type constraint, and allow indexing on the type param. (And make rules more explicit if we disallow certain cases.)

@griesemer @findleyr
@griesemer griesemer added NeedsFix The path to resolution is known, but the work has not been done. release-blocker labels Nov 1, 2021
@griesemer griesemer added this to the Go1.18 milestone Nov 1, 2021
@griesemer
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The current generic indexing is both too relaxed and not tight enough. While this could be made to work, maps are sufficiently different from all the other indexable types that it gets very quickly extremely complicated. We probably should differentiate between all maps and all something else in the type set for now.

@griesemer griesemer changed the title cmd/compile/internal/types2: should handle indexing for typeparam that is map or slice cmd/compile/internal/types2: review implementation of indexing of generic types Nov 1, 2021
@griesemer
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Follow-up: This code is working as intended. If we have a map in the the type set, all types must be maps with the same underlying type, otherwise we cannot index.

What is not working correctly (missing tests!) is assignments to indexed generic expression that are not assignable (because e.g. the type set contains a string). Fix forthcoming.

@gopherbot
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Change https://golang.org/cl/360603 mentions this issue: cmd/compile/internal/types2: fix indexing of generic types

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FrozenDueToAge NeedsFix The path to resolution is known, but the work has not been done. release-blocker
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@danscales @griesemer @gopherbot