error: cannot find `M` in this scope
  --> c.proto:6:8
   |
 6 | extend M {
   |        ^ not found in this scope
   |
   = help: the full name of this scope is `test`

error: cannot find `test.M` in this scope
  --> c.proto:10:8
   |
10 | extend test.M {
   |        ^^^^^^ not found in this scope
   |
   = help: the full name of this scope is `test`

error: cannot find `.test.M` in this scope
  --> c.proto:14:8
   |
14 | extend .test.M {
   |        ^^^^^^^ not found in this scope
   |
   = help: the full name of this scope is `test`

encountered 3 errors
