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The only integer type conversions that are guaranteed to be safe for all data values and all possible conforming implementations are conversions of an integral value to a wider type of the same signedness.

The C Standard, subclause 6.3.1.3 [ISO/IEC 9899:20112024], says

When a value with integer type is converted to another integer type other than _Bool, if the value can be represented by the new type, it is unchanged.

Otherwise, if the new type is unsigned, the value is converted by repeatedly adding or subtracting one more than the maximum value that can be represented in the new type until the value is in the range of the new type.

Otherwise, 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.

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Subclause 6.2.5, paragraph 911, of the C Standard [ISO/IEC 9899:20112024] provides the necessary guarantees to ensure this solution works on a conforming implementation:

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Conversions to signed character types are more problematic.

The C Standard, subclause 6.3.1.3, paragraph 3 [ISO/IEC 9899:20112024], says, regarding conversions

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Furthermore, subclause 6.2.6.2, paragraph 2, says, regarding integer modifications

If the sign Each bit is one, the value shall be modified in one of the following ways:
— the corresponding value with sign bit 0 is negated (sign and magnitude)
— the sign bit has the value −(2M ) (two’s complement);
— the sign bit has the value −(2M − 1) (ones’ complement).
Which of these applies is implementation-defined, as is whether the value with sign bit 1 and all value bits zero (for the first two), or with sign bit and all value bits 1 (for ones’ complement), is a trap representation or a normal value. [See note.]

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that is a value bit shall have the same value as the same bit in the object representation of the corresponding unsigned type. If the sign bit is zero, it shall not affect the resulting value. If the sign bit is one, it has value −(2N−1). There may or may not be any padding bits signed char shall not have any padding bits. 

Consequently, the standard allows for this code to trap:

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Implementations with such trap representations are thought to have existed in the past. Your author was unable to locate any documents describing such processors.

INT31-C-EX3: ISO The C Standard, section subclause 7.2729.2.45, paragraph 3 [ISO/IEC 9899:2024] says:

The time function returns the implementation’s best approximation to the current calendar time. The value (time_t)(−1-1) is returned if the calendar time is not available.

If time_t is an unsigned type, then the expression ((time_t) (-1)) is guaranteed to yield a large positive value.

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