Improve error message regarding proto_path not matching .proto file.

1 files changed, 5 insertions, 1 deletions

diff --git a/src/google/protobuf/compiler/command_line_interface.cc b/src/google/protobuf/compiler/command_line_interface.cc
index 3629f682..3eba3486 100644
--- a/src/google/protobuf/compiler/command_line_interface.cc
+++ b/src/google/protobuf/compiler/command_line_interface.cc
@@ -439,7 +439,11 @@ bool CommandLineInterface::MakeInputsBeProtoPathRelative(
         } else {
           cerr << input_files_[i] << ": File does not reside within any path "
                   "specified using --proto_path (or -I).  You must specify a "
-                  "--proto_path which encompasses this file." << endl;
+                  "--proto_path which encompasses this file.  Note that the "
+                  "proto_path must be an exact prefix of the .proto file "
+                  "names -- protoc is too dumb to figure out when two paths "
+                  "(e.g. absolute and relative) are equivalent (it's harder "
+                  "than you think)." << endl;
         }
         return false;
     }