[NotPaidToPost]

I don't think it would not be simple or a good idea to warn because these are perfectly valid expressions and it would lead to a flood of warnings if compilers started to display "Did you really mean that?" for an ever growing list of expressions (which is what it would become).

Better to leave this to code analysis tools.

Edit:

2^32 uses the ^ operator exactly as intended, and has perfectly legitimate uses, especially when dealing with registers of HW peripherals.

It's not the same as warning on "if (a=b)" as someone replied.

From a language perspective, 2^32 is exactly the same as 2+32.

[rocqua]

It's a warning not an error. Moreover, it is very hard to come up with a legitimate usecase of 2^32 as an expression. People who use it to set bit flags will use &, and for bit flags, octal or hex notation is probably better.

[raxxorrax]

I don't know if it is a good practice, but I often have statements and expressions that look awfully similar. Flipping bits does happen often enough.

Mostly written as y ^ (1 << x), which could easily resolve to these expressions. Mostly at runtime, sure, but there are exceptions. Especially if you like descriptive constants. I would expect it to be quite difficult to separate the "correct cases" without people starting to just suppress compiler warnings or trick it with writing the same stuff in different words (which might be better).

On the other hand, setting max_short to something like 18 is probably a really god prank in larger programming environments. The compiler would just ruin all the fun here.

[derefr]

> Mostly written as y ^ (1 << x), which could easily resolve to these expressions.

We’re talking about a parse-time check; things that resolve to those values after identifier binding won’t throw the warning. It’s not a warning about what you wrote (semantically), it’s a warning about how you wrote it (syntactically).

[rocqua]

The point is very specifically cases where X is a decimal literal (not an expression such as 1 << x, or a hex number).

Things that resolve to this don't trigger the warning, and it is a compile time warning.

And then specifically looking at 2^X and 10^X occurring in source code.

[raxxorrax]

I would agree with the suggestion then. It can happen that you just write it out like that and the compiler should maybe inform me. But why limit it to 2^ and 10^ and not say literal^literal should always produce a warning? Any common or valid usages that I am missing here?

[rocqua]

Certainly, I'd say if either side is a hex or octal literal, it might be someone intentionally bit-twideling.

As for limiting the base, that is a more fluid matter. I guess you want to avoid warnings for very large numbers. Because those are more likely legitimate.

[NotPaidToPost]

> But why limit it to 2^ and 10^ and not say literal^literal should always produce a warning?

You're suggesting that using the C language as specified and intended should produce a warning... That makes no sense.

[pmikesell]

Yes, or 1 << 31, for example.

[rocqua]

That has plenty of usecases.

[NotPaidToPost]

> It's a warning not an error.

Many build environments are set to treat all warnings as errors.

The point is that 2^32 is a perfectly compliant C expression that is neither misleading nor ambiguous, and that also won't create any variable overflow. It uses ^ exactly as intended. Why should the compiler complain? Why should I get a warning/error when following the spec to the letter?

[lifthrasiir]

Like it or not, there are already tons of lint-like warnings in GCC [1] and clang [2]. Many (uh, most?) of them are not required by the C/C++ standards (which by the way have their own requirements for diagnostic messages).

[1] https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html

[2] https://clang.llvm.org/docs/DiagnosticsReference.html

[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.

[stestagg]

Environments where the decision has been made that error-prone, misleading and ambiguous statements should not be allowed typically are those where warning are treated as errors.

A new type of misleading and error-prone statement has been found, so it seems entirely reasonable that this is added to that list.

This seems to be the entire intent of compiler warnings (these days anyway)

[sayusasugi]

Heuristics like this are important because humans are not machines. If you run into a warning in one of these situations, they are usually rectified by changing the statement slightly. Worst case scenario you can #pragma push and pop the warning.

[pjc50]

The compiler should complain because it's overwhelmingly likely to be a mistake.

[viraptor]

You can turn specific warnings off. You can also usually disable them inline. There are very common ways to deal with false positive warnings.

[masklinn]

> It's not the same as warning on "if (a=b)" as someone replied.

"if (a=b)" uses the "=" operator exactly as intended with perfectly legitimate uses. It's warned about because it's an error-prone construct, not because it's incorrect.

Seems to me 2^32 is also error-prone, if you wanted to combine flags you'd use 2|32, using xor here is weird.

[NotPaidToPost]

'If' expects a condition so finding an assignment may be a red flag even if an assignment has a return value in the spec, which makes the construct valid.

On the other hand ^ expects integers so 2^32 is exactly what is expected.

Most the replies I saw here try to second-guess or claim that ^ should be used in a specific way. Not so, ^ is just doing XOR of two integers.

Apparently, I am having an incorrect opinion, though, so I will self-censor and remain silent.

[CJefferson]

You don't have to enable warnings, but most c programmers do.

The problem is, what proportion of 2^32 in C are correct? I'm will to bet it is as close to 0 as doesn't matter. The gcc devs aren't stupid, before enabling a warning like this, they will run a test compiling a substasal chunk of debian, and see if there are any false positives.

[mikeash]

If expects an integer, and = produces an integer when the lhs has integer type.

[blattimwind]

> I don't think it would not be simple or a good idea to warn because these are perfectly valid expressions and it would lead to a flood of warnings if compilers started to display "Did you really mean that?" for an ever growing list of expressions (which is what it would become).

There are many technically valid expressions and statements that will still generate warnings in most compilers.

if(a=b) {} comes to mind first and foremost.

[estebank]

I'm intrigued at what your opinion is about the following Rust errors:

    error[E0308]: mismatched types
     --> src/main.rs:4:8
      |
    4 |     if x = y { println!("eq") }
      |        ^^^^^
      |        |
      |        expected bool, found ()
      |        help: try comparing for equality: `x == y`
      |
      = note: expected type `bool`
                 found type `()`

    error: `<` is interpreted as a start of generic arguments for `u32`, not a comparison
     --> src/main.rs:4:17
      |
    4 |     if i as u32 < 2 {
      |        -------- ^ --- interpreted as generic arguments
      |        |        |
      |        |        not interpreted as comparison
      |        help: try comparing the cast value: `(i as u32)`

[__david__]

For the same reason that this:

    if (c)
        printf("Hello");
        exit(0);
    exit(1);

Produces a warning in gcc:

    test.c:7:5: warning: this ‘if’ clause does not guard... [-Wmisleading-indentation]
         if (c)
         ^~
    test.c:9:9: note: ...this statement, but the latter is misleadingly indented as if it were guarded by the ‘if’
             exit(0);
             ^~~~

"2^32" is of course just an xor, but it makes no sense (the "exclusive" part of it is not being exercised). If you really mean 2 bits being set in a constant then "2 | 32" is much clearer. If you're dead set on xor, "32^2" won't trigger the warning.

[NotPaidToPost]

It's not for you or the compiler to decide that 2^32 makes no sense. It is using ^ exactly as intended and is neither misleading nor ambiguous.

Stating that 32^2 should not trigger a warning while 2^32 should shows that this proposal has not been thought through.

It's not desirable or sensible to raise a warning on the premise that the expression might mean something else in another language, which is what this would do.

[__david__]

I feel perfectly qualified to say that 2^32 makes no sense.

Just because something is specced doesn't mean that it's reasonable. My "if statement" example is perfectly valid C, but it's not reasonable. 2^32 is perfectly valid, but also unreasonable. Frankly, any bitwise operations with >10 decimal constants are either intentionally obfuscating, or done by someone who doesn't know what they are doing. Consider "2^32" vs "0x02^0x20". The latter is much better.

It is entirely sensible to warn on the premise that it might mean something in another language: The construct seems to be an actual point of confusion (the authors sited numerous real code examples where people are accidentally doing this in the wild), and the construct makes no sense at face value. I think you'd be hard pressed to find a real example of 2^32 in code where it isn't a bug. That's enough to make it a reasonable warning.