Comment 1:

I think it's a simple value assignment once one assumes that
type RuntimeInterface struct { /* filtered or unexported fields */ }
Per analogiam, I don't think that the len() and cap() of LHS in `sliceL = sliceR` are
specified to become the "dynamic" values of the RHS. Neither for strings etc.
IMO #WorkingAsIntended

Comment 2:

I don't think you can make the comparison between slices/strings and interfaces. The
assignment of interfaces include an implicit conversion that isn't present on
slice/string assignments.
For example, you can assign an io.WriteCloser to an io.Writer without a cast, even
though they are 2 different types.

Comment 3:

@2: Ah, you're right, the type field is actually not always just copied as it would be
in a simple assignment.

Comment 4:

i think this essentially boils down to the fact that the dynamic type
of interface value will never be another interface.

Comment 5:

Owner changed to @griesemer.

Comment 6:

Labels changed: added go1.3maybe.

Comment 7:

Labels changed: added release-none, removed go1.3maybe.

Comment 8:

Labels changed: added repo-main.

We should try to clarify this soon - it's central to Go. The language around interfaces - while intuitively mostly clear - is imprecise in various ways.

• There's an almost circular definition of what an interface is: The section on interface types refers to the method set specified by the interface and calls it "its interface". The section on method sets defines the method set of an interface as its interface. We don't actually say what the method set is (though it's intuitively clear).

• The section on assignability should be clearer about what assignment of an interface to another interface means.

• Perhaps the definition of "implements" needs to be clarified.

When clarifying this, confirm that #17991 is adequately covered (an any other open issues in this space).

Bumping to Go 1.9, but feel free to send a CL this week for Go 1.8 if you want.