cmd/vet: return analysis fails with type-switching and panic()

[rmmh]

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

go version devel +0c471dfae2 Mon Feb 26 20:22:06 2018 +0000 linux/amd64

Does this issue reproduce with the latest release?

No-- this is in changes after 1.10.

What did you do?

Given a.go:

package kubernetes

func DeepCopyJSONValue(x interface{}) interface{} {
        switch x := x.(type) {
        case []interface{}:
                clone := make([]interface{}, len(x))
                for i, v := range x {
                        clone[i] = v
                }
                return clone
        default:
                panic("cannot deep copy")
        }
}

$ go vet a.go
# command-line-arguments
./a.go:14:1: missing return
vet: typecheck failures

What did you expect to see?

No typecheck failures.

[rmmh]

With go vet becoming part of go test, parsing & typechecking the go corpus might be a good test.

[ysmolsky]

This happened after the 99843e2

% go version ; go vet a.go
go version devel +cbfda7f892 Mon Feb 26 18:32:38 2018 +0000 darwin/amd64
% go version ; go vet a.go
go version devel +99843e22e8 Mon Feb 26 18:46:26 2018 +0000 darwin/amd64
# command-line-arguments
./a.go:14:1: missing return
vet: typecheck failures

/cc @griesemer

[griesemer]

Thanks, @ysmolsky for tracking down the offending change; this made it easy to identify the problem. I've got the fix (minor oversight in 99843e2) and will have a CL later today.

[gopherbot]

Change https://golang.org/cl/97535 mentions this issue: go/types: fix incorrect context when type-checking interfaces