Aliases are supposed to be identical to the type being aliased, so
    questions about type compatibility need to always ignore aliases,
    except for error messages involving the type name.
    
    The test case for this is https://golang.org/cl/94995.
    
    Fixes golang/go#23912
    
    Reviewed-on: https://go-review.googlesource.com/94996

From-SVN: r257845