Tautological compare

Severity: Med ID: tautological-compare

Flags a relational or equality comparison whose two sides are the same expression.

What it does

Reports a <op> a where a is a side-effect-free expression (an identifier, member access, or indexing) and <op> is <, <=, >, >=, ==, or !=. Such a comparison has a constant result. Comparisons whose sides could legitimately differ (for example involving a function call) are left untouched, as are comparisons on user-defined value types, whose operators are user-defined (using {f as ==} for T) and need not be constant.

Why is this bad?

x >= x is always true and x < x is always false, regardless of x. It is almost always a typo for a comparison against a different operand, or leftover dead code that hides a real condition.

Example

Bad

require(balance >= balance); // always true; likely meant another operand
if (a[i] < a[i]) {           // always false; dead branch
    // ...
}

Good

require(balance >= amount);
if (a[i] < a[j]) {
    // ...
}