And this fails to compile, although the docs says it is valid:
// sliceOrMap is a type constraint for a slice or a map.
type sliceOrMap interface {
[]int | map[int]int
}
// Entry returns the i'th entry in a slice or the value of a map
// at key i. This is valid as the result of the operator is always int.
func Entry[T sliceOrMap](c T, i int) int {
// This is either a slice index operation or a map key lookup.
// Either way, the index and result types are type int.
return c[i] // invalid operation: cannot index c (variable of type
T constrained by sliceOrMap
}
On Saturday, November 13, 2021 at 1:10:30 AM UTC+8 tapi...@gmail.com wrote:
> The proposal design docs (
> https://go.googlesource.com/proposal/+/refs/heads/master/design/43651-type-parameters.md)
>
> mentions:
>
> // structField is a type constraint whose type set consists of some
> // struct types that all have a field named x.
> type structField interface {
> struct { a int; x int } |
> struct { b int; x float64 } |
> struct { c int; x uint64 }
> }
>
> // This function is INVALID.
> func IncrementX[T structField](p *T) {
> v := p.x // INVALID: type of p.x is not the same for all types in
> set
> v++
> p.x = v
> }
>
> However, it still fails to compile if all the types of the x fields are
> identical.
>
> type structField interface {
> struct { a int; x int } |
> struct { b int; x int } |
> struct { c int; x int }
> }
>
> func IncrementX[T structField](p *T) {
> v := p.x // p.x undefined (type *T has no field or method x)
> v++
> p.x = v // p.x undefined (type *T has no field or method x)
> }
>
>
>
>
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