Why Doesn't MSVC Warn About Signed/Unsigned Equality Comparisons?

Why MSVC Accepts Signed/Unsigned Equality Comparisons Without Warnings

Despite its strict handling of signed/unsigned mismatches in comparison operators, Microsoft Visual C (MSVC) unexpectedly tolerates equalities between signed and unsigned types. This behavior arises from the underlying conversion rules.

Integral Promotion and Signed/Unsigned Conversion

During comparisons, MSVC follows the integral promotion rules outlined in C Standard 5/9. If one operand is signed and the other unsigned, the signed operand is converted to unsigned.

For equality, this conversion doesn't affect the result: -1 == (unsigned) -1. However, for other comparison operators, it does matter: -1 > 2U is true because -1 is converted to an unsigned value.

MSVC Warning Levels and Signed/Unsigned Comparisons

MSVC's warning levels determine which signed/unsigned mismatches trigger warnings. The developers made specific choices regarding equality vs. greater/less comparisons:

• Equality: -1 == -1 is considered equivalent to -1 == (unsigned) -1, and no warning is issued.
• Greater/Less: -1 < 2 does not equal -1 < (unsigned) 2, and MSVC generates a warning for these comparisons.

This approach reflects the developers' view that equality comparisons involving signed/unsigned types are often intentional, while greater/less comparisons with mixed types are more likely to lead to errors.

In conclusion, MSVC's silent acceptance of signed/unsigned equality comparisons stems from integral promotion rules and the compiler's specific warning configuration for such scenarios.

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