This switch tells the assembler that any Warning messages are to be treated as though they were errors; this causes the assembler to end with a non-zero exit code, and helps prevent any warning conditions from "passing by" unnoticed.

┌────┬──────┬─────┬────┬──────────────────────────────────────────────────┐
│Type│Global│Group│File│Default                                           │
├────┼──────┼─────┼────┼──────────────────────────────────────────────────┤
│S   │Yes   │Yes  │Yes │-Mwe (warnings are not considered to be errors)   │
└────┴──────┴─────┴────┴──────────────────────────────────────────────────┘