protoc: explicit map_entry option is sometimes allowed

[jhump]

In the following example source file, protoc correctly produces an error:

// test.proto
message Foo { option map_entry = true; reserved 10 to 1; }
message Bar { optional Foo foo = 1; }

> protoc test.proto -o /dev/null
test.proto:3:24: map_entry should not be set explicitly. Use map<KeyType, ValueType> instead.

Interestingly, the reported position in the error message is where the message is used, not where it is defined.

However, if the file does not actually refer to the offending message type (Foo), the compiler accepts the invalid input:

// test.proto
message Foo { option map_entry = true; reserved 10 to 1; }

With the above, running protoc test.proto -o /dev/null produces no error.

This is inconsistent, but it also leads to "errors at a distance". If I define a file elsewhere that uses this message, I get an error in that other compile operation, instead of in the original:

// test2.proto
import "test.proto";
message Bar { optional Foo foo = 1; }

> protoc test2.proto -o /dev/null
test2.proto:3:24: map_entry should not be set explicitly. Use map<KeyType, ValueType> instead.

[fowles]

Also a bug, feel free to send a PR.