tree d0694dd3cce2aebb061d6515dd55a397c27e1e10
parent ebc699d09917d3b867eb08fe8470657c3a355592
author Joe Tsai <joetsai@digital-static.net> 1536776775 -0700
committer Joe Tsai <thebrokentoaster@gmail.com> 1536868331 +0000

reflect/protoreflect: add Has and Clear to KnownFields and Map

Change the API to use explicit Has/Clear methods instead of relying on the
"invalid" form of Value to represent nullability.

There are several reasons for this change:

* Much of the ecosystem around protobufs do not care about nullability alone.
For example, for the encoder to determine whether to emit a field:
it needs to first check if a field is nulled, and if not, it still needs to go
through a series of type-assertion to check whether the value is the zero value.
It is much easier to be able to just call Has.

* It enables representing the default value as part of the value API, rather
than only as part of the descriptor API.

* The C++ API also uses explicit has and clear methods. However, we differ from
them by defining Has for proto3 scalars (while C++ panics instead).

For internal consistency, we also use a Has/Clear API for Maps.

Change-Id: I30eda482c959d3e1454d72d9fc761c761ace27a6
Reviewed-on: https://go-review.googlesource.com/134998
Reviewed-by: Damien Neil <dneil@google.com>