Dave Fladebo noticed the changes to enforce readable/writable/seekable in std.stream for dmd.120 seems to break some uses of std.stream.stdin like eof() and readLine() and other (important) things. I'll send Walter an update for the next dmd release but if you need stdin to work I recommend recompiling phobos with the assert in the function assertSeekable commented out. That will revert back to the dmd-119 behavior of treating stdin and stdout as seekable.

Sorry for the inconvenince!
-Ben