This only happens if the field is unexported. If it's exported, String is properly called:

https://play.golang.org/p/STKBOnEVrb

That was mentioned in the original "golang-nuts" thread quoted earlier.
But I don't see why the exported-ness of a field should affect whether String() is used or not to print that field.

works as expected.

reflect can not extract the concrete value of an unexported field and thereby fmt cant either to call the String method.
https://play.golang.org/p/JPUA_FHkov

related:
#8965

If there are no arguments against i think this can be closed.

That the String method of an unexported field can not be called in another package is normal behavior and fmt is another package. In general e.g. somestruct.someunexportedfield.String() is not directly callable in another package.

It is possible to define a String method on Box that has access to elem and then hands this over to fmt directly to allow for the unexported fields String method to be called:

func (b Box) String() string { return fmt.Sprintf("%s", b.elem) }

func (i Int) String() string { return fmt.Sprintf("Int(%d)", int(i)) }

I wonder if we can document this in some way that makes sense.

CL https://golang.org/cl/31817 mentions this issue.