dev.go2go: translator generates incorrect code for type switches, leading to duplicate cases

What version of Go are you using (go version)?

$ go version
go version devel +165ceb09f9 Tue Nov 3 12:31:40 2020 -0500 windows/amd64

Does this issue reproduce with the latest release?

n/a

What operating system and processor architecture are you using (go env)?

• windows/amd64
• go2go playground

What did you do?

https://go2goplay.golang.org/p/dTkkesE4GCX.
Error is printing when go1 compiler tries to compile translated code.
Also I ran on dev.typeparams

go tool compile -G ./issue.go

where issue.go is code from playground

Here instantiated createRequest[io.Writer] look something like:

func createRequest_ioWriter(w io.Writer) {
	switch (interface{})(w).(type) {
	case io.Writer:
		fmt.Println("W")
	case io.Writer:
		fmt.Println("io.Writer")
	}

}

What did you expect to see

An error from go2go like type parameter cannot be case of type switch.

What did you see instead?

Code passes go2go and dev.typeparams type checker.

CC @griesemer

[griesemer]

The typeparams draft doesn't disallow type parameters in type switch cases (or type assertions for that matter). A type parameter is a type like any other, so it's not obvious that this should be prohibited.

At the moment I consider this a limitation of the source-to-source translator (prototype).

[ianlancetaylor]

I agree, but I don't plan to do any more work on this experimental tool. If anybody else wants to send a fix, that would be fine.

[gopherbot]

Change https://golang.org/cl/273127 mentions this issue: [dev.go2go] go/types, cmd/compile/internal/types2: a type parameter is a valid type case in a type switch

[gopherbot]

Change https://golang.org/cl/273128 mentions this issue: [dev.typeparams] cmd/compile/internal/types2: a type parameter is a valid type case in a type switch

[griesemer]

Just for the record: The CLs above don't address this issue - they only clean up related handling of type parameters in type switches (which are carried forward into the dev.typeparams branch).