Interface PHPStan\Type\CompoundType

Marker interface for types that require bidirectional type comparison.

Simple types like StringType or IntegerType can answer isSuperTypeOf() and accepts() on their own — they check whether the incoming type fits. But compound types (unions, intersections, mixed, never, accessory types, integer ranges, callables, iterables, conditionals, etc.) need to be asked from the other direction, because they carry internal structure that the simple type on the other side knows nothing about.

The protocol works like a double dispatch:

1. A simple type's accepts()/isSuperTypeOf() receives an argument.
2. It checks if ($type instanceof CompoundType).
3. If true, it delegates to $type->isAcceptedBy($this, …) or $type->isSubTypeOf($this).
4. The compound type then decomposes itself (e.g., iterates union members) and calls back to the simple type for each component.

This avoids the simple type having to understand union/intersection/mixed/never semantics. For example, StringType::accepts() doesn't need to know how to check a UnionType<string|int> — it just delegates to UnionType::isAcceptedBy(), which iterates its members and asks StringType::accepts() for each one.

Unlike instanceof SomeSpecificType checks (which are discouraged in CLAUDE.md), instanceof CompoundType is the correct and intended pattern throughout the type system. It is part of the double-dispatch protocol, not a type query.

Implementations include:

• UnionType — isSubTypeOf() requires ALL members to be subtypes, isAcceptedBy() requires ALL to be accepted
• IntersectionType — isSubTypeOf() requires at least ONE member to be a subtype (via maxMin)
• MixedType, NeverType — terminal cases (mixed accepts everything, never is subtype of everything)
• All AccessoryType implementations — refinement types that live inside intersections
• IntegerRangeType, CallableType, IterableType — types with internal structure
• ConditionalType, KeyOfType, ValueOfType, etc. — late-resolvable types