Generated Kotlin codes for optional fields in proto3 is Kotlin not-nullable fields, and they're better to be nullable fields.

[chuanwise]

To read the optional field foo in Kotlin:

optional int64 foo = 1;

Because the generated codes are like:

@java.lang.Override
public boolean hasFoo() { ... }

@java.lang.Override
public long getFoo() { ... }

We need to bar.foo.takeIf { bar.hasFoo() }, which is not Kotlin-friendly. It's expected to add @Nullable or @NotNull to getter methods.

[nillpoe]

This should be filed against the protobuf project.