The GC setup code assumes that if non of the Use*GC flags is true we should be using SerialGC. But in that case we don't set UseSerialGC to true.

For example running with -client will enable the SerialGC without setting UseSerialGC to true:

java -client -XX:+PrintFlagsFinal -version | grep "UseSerialGC\|UseG1GC\|UseParallelGC\|UseConcMarkSweepGC"
     bool UseConcMarkSweepGC = false {product}
     bool UseG1GC = false {product}
     bool UseParallelGC = false {product}
     bool UseSerialGC = false {product}
java version "1.9.0-ea"
Java(TM) SE Runtime Environment (build 1.9.0-ea-b44)
Java HotSpot(TM) Client VM (build 1.9.0-ea-b44, mixed mode)

The GC code can be simplified if we know that one of UseSerialGC, UseG1GC, UseParallelGC and UseConcMarkSweepGC is always true.