[barrkel]

Plenty of valid expressions generate warnings. A frequent gotcha is `=` in boolean contexts. Warnings are justified, and can normally be avoided with a little bit more lexical work (e.g. an extra set of parentheses). The upside is large and the downside is miniscule.

[OskarS]

Isn't that pretty much the definition of a warning? An expression that is technically valid, but probably a mistake. If something wasn't a valid expression, it would be a compiler error, not a warning.

[mikeash]

Usually, yes. C is weird because undefined behavior is illegal, can sometimes be detected statically, and yet will not produce an error. So sometimes you get warnings for invalid yet error-free code. It’s a strange language.