go/types, types2: handle embedding of type parameters correctly

[griesemer]

package p

func _[E1 interface{ m() E1 }, E2 interface{ E1 } ](c1 chan E1, c2 chan E2) {
        x1 := (<-c1).m()
        x2 := (<-c2).m()
        _ = x1
        _ = x2
}

produces

(<-c2).m undefined (type bound for E2 has no method m)

This should either be permitted or embedding of E1 in the E2 constraint should be explicitly forbidden.

cc: @findleyr

[ianlancetaylor]

I thought we had agreed to not support embedding of plain type parameters, and to only support embedding of type literals that refer to type parameters. Am I misremembering?

[griesemer]

I believe you're correct. Yesterday I filed whatever looked like it needed attention. Clearly this needs attention as in: report an error at the embedding location.

[gopherbot]

Change https://golang.org/cl/334151 mentions this issue: [dev.typeparams] cmd/compile/internal/types2: embedding stand-alone type parameters is not permitted

[gopherbot]

Change https://golang.org/cl/335082 mentions this issue: [dev.typeparams] go/types: embedding stand-alone type parameters is not permitted