Why is this an issue?

The use of operators pairs ( =+, =- or =! ) where the reversed, single operator was meant (+=, -= or !=) will compile and run, but not produce the expected results.

This rule raises an issue when =+, =-, or =! is used without any spacing between the two operators and when there is at least one whitespace character after.

Noncompliant code example

var target, num = -5, 3

target =- num  // Noncompliant; target = -3. Is that really what's meant?
target =+ num // Noncompliant; target = 3

Compliant solution

var target, num = -5, 3

target = -num  // Compliant; intent to assign inverse value of num is clear
target += num