Rule 12.7 and underlying type

[misra-c]

MISRA-C meeting 22-8-2006

The array index sub-expression

Code:

var2 - 1

contains both unsigned and signed values (respectively) and is prohibited under rule 10.1.

The underlying type of a mixed signed / unsigned expression is undefined.

In your second example, the array index sub-expression

Code:

var2 - 1U

is unsigned, and therefore the shift can be applied.