Why Doesn't MSVC Warn About Signed/Unsigned Equality Comparisons (==) But Warns About Other Comparisons?

Comparison of Signed and Unsigned Values in MSVC: Understanding the Absence of Warnings

In this code snippet, you notice the absence of warnings for a specific comparison:

int a = INT_MAX;
unsigned int b = UINT_MAX;
bool c = false;
...
if (a == b) // no warning

This raises the question of why the comparison between signed and unsigned values doesn't trigger a warning when compared using ==, while it does for the other comparisons (<, >, <=, >=).

Background Promotion and C4018

You expected this comparison to produce a warning because of background promotion. However, the last two examples refute this logic:

if (((unsigned int)a) == b) // no warning
if (a == ((int)b)) // no warning

Conversion Rules

When comparing signed and unsigned values, the compiler follows specific conversion rules outlined in the C standard. For equality comparison (==), it doesn't matter if the operands are signed or unsigned because the result is always a boolean (true or false).

However, for other comparisons, the signed value is converted to unsigned. This conversion is significant because it affects the comparison results. For instance, in the following statement:

-1 > 2U

The result is true because -1 is converted to a large positive unsigned integer, resulting in a correct comparison.

MSVC Warning Levels

MSVC's choice to suppress warnings for signed/unsigned equality comparisons is intentional. According to the developers, comparing signed and unsigned values using == often results in an intuitive result. For example, -1 == -1 is true regardless of whether they are signed or unsigned.

On the other hand, comparisons involving greater than (>) or less than (<) on signed/unsigned combinations are more likely to lead to unexpected results. Therefore, MSVC issues warnings for these comparisons at an earlier stage of optimization to help developers catch potential issues.

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