go/token's FileSet AddFile is:

func (s *FileSet) AddFile(filename string, base, size int) *File

But the "base" parameter is typically from s.Base().

(*FileSet).Base() is protected by a mutex, but one that is locked and unlocked before
AddFile also acquires it.

That means two goroutines can allocate the same base and blow up in
go/parser.Parser.AddFile -> token.FileSet.AddFile.

If FileSet's AddFile's base accepted a negative number to mean "you allocate the
base, from fs.Base()", then it could be under the same lock and the higher-level
race would be removed.

And it wouldn't be changing the API.