Shafik Yaghmour

The value of d is indeterminate.

You may expect it to be undefined behavior but the standard provides for several exceptions for when using an indeterminate value is not undefined behavior, see [basic.indet]p2:

if an indeterminate value is produced by an evaluation, the behavior is undefined except in the following cases:

…

• If an indeterminate value of unsigned ordinary character type or std::byte type is produced by the evaluation of the right operand of a simple assignment operator ([expr.ass]) whose first operand is an lvalue of unsigned ordinary character type or std::byte type, an indeterminate value replaces the value of the object referred to by the left operand.

…

 [Example:

int f(bool b) {
   unsigned char c;
   unsigned char d = c;          // OK, d has an indeterminate value
   int e = d;                    // undefined behavior
   return b ? d : 0;             // undefined behavior if b is true
}

— end example]

This was clarified because of defect report 1787 also see GB comment 2 And my Stackoverflow answer which also covers some C background.