TypeVariance enum

Represents the variance annotations for generic type parameters in Dart.

Variance defines how generic type parameters behave with respect to subtyping:

• Covariant (out): Preserves subtyping direction
• Contravariant (in): Reverses subtyping direction
• Invariant: Neither covariant nor contravariant

{@template type_variance_features}

Values

• covariant: Marked with covariant keyword or out in some languages
• contravariant: Marked with in keyword in some languages
• invariant: Default variance with no keyword

Dart Usage

In Dart, variance is primarily expressed through:

• covariant keyword for parameters
• Method parameter positions (contravariant)
• Default invariant behavior

Examples

// Covariant type parameter
class Box<out T> {
  T get value => ...;
}

// Contravariant type parameter  
class Consumer<in T> {
  void accept(T value) {...}
}

// Invariant type parameter
class Holder<T> {
  T value;
}

{@endtemplate}