Annotates a "top-level" type-variable usage that takes its nullness from the type argument supplied by the user of the class. For example, Multiset.Entry.getElement() returns @ParametricNullness E, which means:

• getElement on a Multiset.Entry<@NonNull String> returns @NonNull String.
• getElement on a Multiset.Entry<@Nullable String> returns @Nullable String.

This is the same behavior as type-variable usages have to Kotlin and to the Checker Framework. Contrast the method above to:

• methods whose return type is a type variable but which can never return null, typically because the type forbids nullable type arguments: For example, ImmutableList.get returns E, but that value is never null. (Accordingly, ImmutableList is declared to forbid ImmutableList<@Nullable String>.)
• methods whose return type is a type variable but which can return null regardless of the type argument supplied by the user of the class: For example, ImmutableMap.get returns @Nullable E because the method can return null even on an ImmutableMap<K, @NonNull String>.

Consumers of this annotation include:

• Kotlin, for which it makes the type-variable usage (a) a Kotlin platform type when the type argument is non-nullable and (b) nullable when the type argument is nullable. We use this to "undo" ElementTypesAreNonnullByDefault. It is the best we can do for Kotlin under our current constraints.
• NullAway, which will treat it identically to Nullable as of version 0.9.9. To treat it that way before then, you can set -XepOpt:NullAway:CustomNullableAnnotations=com.google.common.base.ParametricNullness,...,com.google.common.util.concurrent.ParametricNullness, where the ... contains the names of all the other ParametricNullness annotations in Guava. Or you might prefer to omit Guava from your AnnotatedPackages list.
• J2ObjC
• NullPointerTester, at least in the Android backport (where the type-use annotations NullPointerTester would need are not available) and in case of JDK-8202469

This annotation is a temporary hack. We will remove it after we're able to adopt the JSpecify nullness annotations and tools no longer need it.