The key property here is what the bit pattern represents,
not what its type is. Storing 5 into a pointer is the problem.
Storing a uintptr that holds pointer bits back into a pointer
is not as much of a problem, and not what we are claiming
the runtime will detect.

Longer discussion at
https://groups.google.com/d/msg/golang-nuts/dIGISmr9hw0/0jO4ce85Eh0J

LGTM=r
R=r
CC=golang-codereviews
https://golang.org/cl/98370045