go/types, types2: type checker does not allow to use receive operation even all types in type list are channels

[tdakkota]

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

$ go version
go version devel +f065ff221b Tue Jan 12 15:56:55 2021 -0800 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/4ERkDYXAVM-
Also, I got same result from dev.typeparams typechecker using

$ go tool compile -G ./example.go

The problem is here

go/src/cmd/compile/internal/types2/expr.go

Lines 156 to 172 in f065ff2

	case syntax.Recv:
	typ := x.typ.Chan()
	if typ == nil {
	check.invalidOpf(x, "cannot receive from non-channel %s", x)
	x.mode = invalid
	return
	}
	if typ.dir == SendOnly {
	check.invalidOpf(x, "cannot receive from send-only channel %s", x)
	x.mode = invalid
	return
	}
	x.mode = commaok
	x.typ = typ.elem
	check.hasCallOrRecv = true
	return
	}

x.typ.Chan() returns nil if x.typ is a *TypeParam.

What did you expect to see?

Successful type-check or a stub error like generic slice expressions not yet implemented. All types in RecvChan[T] type list support receive operation.

What did you see instead?

An error

invalid operation: cannot receive from non-channel ch (variable of type parameter type C)

[tdakkota]

CC @griesemer

[griesemer]

This has not yet been implemented.

[rogpeppe]

I was about to file an issue about this when I found this one.

I'll also mention that type inference doesn't work when the type parameters are omitted from Recv, but I don't see any particular reason why it shouldn't.

[griesemer]

@rogpeppe Type inference does work if you remove one of the channel types from RecvChan. That is because constraint type inference requires exactly one type in the type list/set. Channels with channel directions are a bit special; they may need some special treatment. I filed a separate issue (#45920).

[gopherbot]

Change https://golang.org/cl/333709 mentions this issue: [dev.typeparams] cmd/compile/internal/types2: implement <-ch where ch is of type parameter type

[gopherbot]

Change https://golang.org/cl/335076 mentions this issue: [dev.typeparams] go/types: implement <-ch where ch is of type parameter type

[gopherbot]

Change https://golang.org/cl/335075 mentions this issue: [dev.typeparams] go/types: implement <-ch where ch is of type parameter type

[griesemer]

This was fixed with the above CLs.