go/types, types2: conversions involving type parameters don't work

[griesemer]

Reminder issue that conversions involving type parameters are implemented ad-hoc and don't work as expected.

For instance (https://go2goplay.golang.org/p/8MHhIOoWA29):

package p

type Int interface{ type int }
type String interface{ type string }
type IntOrString interface {
	type int, string
}

func Convert1[Dst Int, Src Int](x Src) Dst            { return Dst(x) }
func Convert2[Dst String, Src Int](x Src) Dst         { return Dst(x) }
func Convert3[Dst String, Src String](x Src) Dst      { return Dst(x) }
func Convert4[Dst IntOrString, Src Int](x Src) Dst    { return Dst(x) }
func Convert5[Dst String, Src IntOrString](x Src) Dst { return Dst(x) }

(test case by @mdempsky)

See also the section on type conversions in the generics proposal.

cc: @findleyr

[toothrot]

Checking in on this issue as it's labeled a release blocker for Go 1.18. Is there any update?

[griesemer]

This will be worked on soon.

[griesemer]

Updated test cases.

package p

func convert1[Dst ~int, Src ~int](x Src) Dst            { return Dst(x) }
func convert2[Dst ~string, Src ~int](x Src) Dst         { return Dst(x) }
func convert3[Dst ~string, Src ~string](x Src) Dst      { return Dst(x) }
func convert4[Dst ~int|~string, Src ~int](x Src) Dst    { return Dst(x) }
func convert5[Dst ~string, Src ~int|~string](x Src) Dst { return Dst(x) }

[griesemer]

@ianlancetaylor I am certainly expecting convert1 and convert3 to work, but I'm not sure that we need to support string(int) conversions that are generic, specifically in light of #3939. Comments?

[ianlancetaylor]

Seems fine to me. But presumably these will work:

func convert6[Dst ~string, Src ~byte](x Src) Dst         { return Dst(x) }
func convert7[Dst ~string, Src ~rune](x Src) Dst         { return Dst(x) }

[griesemer]

Yes, those will work. Should have a CL with all expected conversions soon.

[gopherbot]

Change https://golang.org/cl/356010 mentions this issue: cmd/compile/internal/types2: implement generic conversions

[gopherbot]

Change https://golang.org/cl/357333 mentions this issue: cmd/compile/internal/types2: use correct types when checking generic conversions

[gopherbot]

Change https://golang.org/cl/357249 mentions this issue: cmd/compile/internal/types: more detailed error messages for generic conversions