This is untrue; see section 6.3.1.3: When a value with integer type is converted to another integer type ... [if] the new type is signed and the value

This is untrue; see section 6.3.1.3: When a value with integer type is converted to another integer type ... [if] the new type is signed and the value cannot be represented in it; either the result is implementation-defined or an implementation-defined signal is raised. http://www.open-std.org/jtc1/sc22/WG14/www/docs/n1570.pdf http://www.open-std.org/jtc1/sc22/WG14/www/docs/n1570.pdf So casts from unsigned to signed must have a well-defined result, but the implementation can choose how to define it. You could in theory have an implementation that defines that casts from unsigned to signed are always positive,[1] but since that's not how GCC and every other mainstream (existing?) compiler defines it, they are not allowed to assume the result of a cast from an arbitrary unsigned integer to a signed integer of equal width is positive. [1] In which case the compiler would be allowed to assume that unsigned-to-signed casts are never negative. This assumption would however be both safe and correct, and would have nothing to do with UB.