Enum InferencePriority 

pub enum InferencePriority {
    NakedTypeVariable = 1,
    HomomorphicMappedType = 2,
    PartialHomomorphicMappedType = 4,
    MappedType = 8,
    ContravariantConditional = 16,
    ReturnType = 32,
    LowPriority = 64,
    Circular = 128,
}

Priority levels for generic type inference constraints.

TypeScript uses a multi-pass inference algorithm where constraints are processed in priority order. Higher priority constraints (like explicit type annotations) are processed first, then lower priority constraints (like contextual types from return position) are processed in subsequent passes.

This prevents circular dependencies and any leakage in complex generic scenarios like Array.prototype.map or Promise.then.

Priority Order (Highest to Lowest)

1. NakedTypeVariable - Direct type parameter with no constraints (highest)
2. HomomorphicMappedType - Mapped types that preserve structure
3. PartialHomomorphicMappedType - Partially homomorphic mapped types
4. MappedType - Generic mapped types
5. ContravariantConditional - Conditional types in contravariant position
6. ReturnType - Contextual type from return position (low priority)
7. LowPriority - Fallback inference (lowest)
8. Circular - Detected circular dependency (prevents infinite loops)

Example

function map<U>(arr: T[], fn: (x: T) => U): U[];
// When calling map(x => x.toString()):
// 1. T is inferred from array element type (NakedTypeVariable)
// 2. U is inferred from return type contextual type (ReturnType)
// Processing T first prevents circular T <-> U dependency

Part of the Priority-Based Contextual Inference implementation.

Variants

NakedTypeVariable = 1

Naked type variable with no constraints (highest priority). Example: <T> where T appears directly in parameter types.

HomomorphicMappedType = 2

Mapped type that preserves array/tuple structure. Example: Partial<T[]> preserves array structure.

PartialHomomorphicMappedType = 4

Partially homomorphic mapped type. Example: Mapped types with some mixed properties.

MappedType = 8

Generic mapped type. Example: { [K in keyof T]: U }

ContravariantConditional = 16

Conditional type in contravariant position. Example: Inference from function parameter types in conditional types.

ReturnType = 32

Contextual type from return position. Example: const x: number = fn() where fn is generic.

LowPriority = 64

Low priority fallback inference.

Circular = 128

Detected circular dependency (prevents infinite loops). Set when a type parameter depends on itself through constraints.

Implementations

impl InferencePriority

pub const NORMAL: Self = Self::ReturnType

Default priority for normal constraint collection.

pub const HIGHEST: Self = Self::NakedTypeVariable

Highest priority for explicit type annotations.

pub const LOWEST: Self = Self::LowPriority

Lowest priority for fallback inference.

pub fn should_process_in_pass(&self, current_pass: Self) -> bool

Check if this priority level should be processed in a given pass.

Multi-pass inference processes constraints in increasing priority order. Returns true if this priority matches or is lower than the current pass level.

pub const fn next_level(&self) -> Option<Self>

Get the next priority level for multi-pass inference.

Trait Implementations

impl Clone for InferencePriority

fn clone(&self) -> InferencePriority

Returns a duplicate of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Debug for InferencePriority

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl Hash for InferencePriority

fn hash<__H: Hasher>(&self, state: &mut __H)

Feeds this value into the given Hasher.

fn hash_slice<H>(data: &[Self], state: &mut H)

where H: Hasher, Self: Sized,

Feeds a slice of this type into the given Hasher.

impl Ord for InferencePriority

fn cmp(&self, other: &InferencePriority) -> Ordering

This method returns an Ordering between self and other.

fn max(self, other: Self) -> Self

where Self: Sized,

Compares and returns the maximum of two values.

fn min(self, other: Self) -> Self

where Self: Sized,

Compares and returns the minimum of two values.

fn clamp(self, min: Self, max: Self) -> Self

where Self: Sized,

Restrict a value to a certain interval.

impl PartialEq for InferencePriority

fn eq(&self, other: &InferencePriority) -> bool

Tests for self and other values to be equal, and is used by ==.

fn ne(&self, other: &Rhs) -> bool

Tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

impl PartialOrd for InferencePriority

fn partial_cmp(&self, other: &InferencePriority) -> Option<Ordering>

This method returns an ordering between self and other values if one exists.

fn lt(&self, other: &Rhs) -> bool

Tests less than (for self and other) and is used by the < operator.

fn le(&self, other: &Rhs) -> bool

Tests less than or equal to (for self and other) and is used by the <= operator.

fn gt(&self, other: &Rhs) -> bool

Tests greater than (for self and other) and is used by the > operator.

fn ge(&self, other: &Rhs) -> bool

Tests greater than or equal to (for self and other) and is used by the >= operator.

impl Copy for InferencePriority

impl Eq for InferencePriority

impl StructuralPartialEq for InferencePriority