DMD: Why bytes, etc. are promoted to ints (if not D linkage)

[Luhrel]

Related to: https://issues.dlang.org/show_bug.cgi?id=20644

src/dmd/expressionsem.d: 2081
---
if (tf.linkage != LINK.d)
{
     // Promote bytes, words, etc., to ints
     arg = integralPromotions(arg, sc);
---

What was to purpose of this line ?

Was there some incompatibilities between a C byte and a D byte 
(or short, or whatever) ?