Rule 6-2-1 Assignment operators shall not be used in subexpressions

[nishiyama]

Hello.

Why are the following MISRA Example compliant?
Is not a sub-expression?

Code:

if ( int16_t i = foo ( ) ) // Compliant
{
}

It looks like the following, but the following are non-compliant.
What are the differences?

Code:

if ( x = y ) // Non-compliant
{
　foo ( );
}