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## Type-Based Tautology

**Severity**: `Med`
**ID**: `type-based-tautology`

### What it does

Flags comparisons that are always true or false because of an integer type's range,
such as `uint256 x >= 0`. Also reports conditions that cover the entire range, such as
`x > 0 || x == 0` for unsigned `x`.

### Why is this bad?

A condition that is permanently true contributes no useful logic and may hide a bug where the developer intended to compare against a different value or use a differently sized type. A condition that is permanently false creates unreachable code, which can silently suppress intended behavior such as access control checks or error handling.

### Example

```solidity
function isValid(uint256 x) public pure returns (bool) {
    return x >= 0; // always true, uint cannot be negative
}

function isInRange(uint8 x) public pure returns (bool) {
    return x < 256; // always true, uint8 max is 255
}

function isBelowMin(int8 x) public pure returns (bool) {
    return x < -128; // always false, int8 min is -128
}

function isImpossible(uint8 x) public pure returns (bool) {
    return x == 256; // always false, 256 is outside uint8 range
}

function coversRange(uint256 x) public pure returns (bool) {
    return x > 0 || x == 0; // always true, every uint256 is either zero or greater
}
```

Use instead:

```solidity
function isValid(uint256 x) public pure returns (bool) {
    return x > 0; // meaningful: false when x == 0
}

function isInRange(uint8 x, uint8 limit) public pure returns (bool) {
    return x < limit; // compare against a runtime value
}

function isBelowThreshold(int8 x) public pure returns (bool) {
    return x < -100; // a value within the representable range
}
```