https://play.golang.org/p/tscYiZK493z
Works but is unnecessarily complex to do something so basic.

A multi-value tuple can't mix with other values as one argument list in a function call (or the source value list in an assignment).
And tuples are not slices.

This language behavior is designed and intended.

We don't use the issue tracker to discuss language issues. Use a forum. See https://golang.org/wiki/Questions. Thanks.

A multi-value tuple can't mix with other values as one argument list in a function call (or the source value list in an assignment).
And tuples are not slices.

I believe it is converted to a slice implicitly. Hence why the first example works as shown in the code fragment on Playground.
So your comment "And tuples are not slices." seems irrelevant and inaccurate at best.

The types of all elements of a slice are the same one,. For a multi-value expression, it is not true.

Yes, for the workable case (https://play.golang.org/p/JNs31SQmAXc), a multi-value expression (tuple) is expanded to multiple single values and these single values are then converted (appended) into a slice. You can also think the slice is converted from the tuple directly, but this is not the key of this issue. Actually, compilers may also convert (a multi-value expression + other value expressions) shown in the unworkable cases to a slice, if Go specification allows such conversions.

There are surely some reasons why the spec prohibits them. You can look for the reasons in some Go forms/groups. The current rules might be some over-stricter for some cases. If you do think the rules should be relaxed, you could make a formal proposal instead of reporting it as a bug.