Type Conversion Error with Empty and Non-Empty Generic Types in Record Mapping

[malcabarzel]

🔎 Search Terms

"empty object", "generic", "map", "infer", "two", "conversion error", "partial"

🕗 Version & Regression Information

This is the behavior in every version I tried, and I reviewed the FAQ for entries about generic type inference with empty object types.

⏯ Playground Link

example-two-types-error

example-single-type-no-error

💻 Code

type ExampleModel = {
    fieldA: number;
    fieldB: string;
};

/**
 * Generic type with two type parameters.
 * Represents a mapping structure for two types (T1 and T2).
 * In real use cases, 'any' would be replaced with converter functions for each type's field.
 */
type TypesMapper<T1, T2> = {
    mapT1: { [K in keyof T1]: any };
    //mapT1: { [K in keyof Partial<T1>]: any };  // Changing to Partial<T1> removes the error only if at least one mapper has a non-empty T1 (mapperWithEmptyT2 is uncommented)
    
    mapT2: { [K in keyof T2]: any };  
    //mapT2: { [K in keyof Partial<T2>]: any };  // Changing to Partial<T2> removes the error only if at least one mapper has a non-empty T2 (mapperWithEmptyT1 is uncommented)
};

// Declare mappings
declare const mapperWithEmptyT1: TypesMapper<{}, ExampleModel>;
declare const mapperWithEmptyT2: TypesMapper<ExampleModel, {}>;
declare const mapperWithBothTypes: TypesMapper<ExampleModel, ExampleModel>;
declare const mapperWithEmptyTypes: TypesMapper<{}, {}>;

const MAPPERS = {
    // Uncomment different cases to observe behavior:
    //mapperWithBothTypes,    // ✅ Prevents error (because both T1 and T2 are non-empty)
    //mapperWithEmptyTypes,   // ✅ Prevents error (because both T1 and T2 are empty)
    mapperWithEmptyT1,       // ❌ Causes error (because T1 is empty and T2 is not)
    mapperWithEmptyT2,       // ❌ Causes error (because T2 is empty and T1 is not)
} satisfies Record<string, TypesMapper<any, any>>;

// Extracting types
type MapperKeys = keyof typeof MAPPERS;

type InferT1<Key extends MapperKeys> = typeof MAPPERS[Key] extends TypesMapper<infer T1, infer T2> ? T1 : never;
type InferT2<Key extends MapperKeys> = typeof MAPPERS[Key] extends TypesMapper<infer T1, infer T2> ? T2 : never;

export type MapperFromKey<Key extends MapperKeys> = TypesMapper<InferT1<Key>, InferT2<Key>>;

// Function to access a mapper by name
const getMapperByKey = <Key extends MapperKeys>(key: Key): MapperFromKey<Key> => {
    // ❌ TypeScript error occurs here when MAPPERS contains only objects with one empty generic type and one non-empty generic type
    // ✅ The error disappears if Partial<T1> or Partial<T2> is used AND at least one mapper has a non-empty type for the partial parameter.
    return MAPPERS[key] as MapperFromKey<Key>;
};

🙁 Actual behavior

When creating a type with two generic types, where each generic type has a property that is an object containing all the type’s fields with some assigned type, an issue arises when using a Record<string, any> (as a map) that stores these types as values.

If all the mapped values have one generic type as an empty type ({}) and the other as a non-empty type, TypeScript produces a type conversion error when using a generic function to retrieve values from the map with inferred types, based on a key name type (as shown in the steps below).

The error disappears when:

• Adding a type where both generic types are empty (mapperWithEmptyTypes).
• Adding a type where both generic types are non-empty (mapperWithBothTypes).
• Applying the workaround described below using Partial<T1> or Partial<T2>.

When performing the steps below, the error occurs on line 45:

return MAPPERS[key] as MapperFromKey<Key>;

Error Message:
Conversion of type 'TypesMapper<{}, ExampleModel> | TypesMapper<ExampleModel, {}>' to type 'MapperFromKey' may be a mistake because neither type sufficiently overlaps with the other.
If this was intentional, convert the expression to 'unknown' first.
Type 'TypesMapper<ExampleModel, {}>' is not comparable to type 'MapperFromKey'.
Types of property 'mapT2' are incompatible.
Type '{}' is not comparable to type '{ [K in keyof InferT2]: any; }'.(2352)

Steps that Reproduce the Issue (with the Attached Code)

1. Using only mapperWithEmptyT1 and/or mapperWithEmptyT2 causes an error

• Comment mapperWithBothTypes and mapperWithEmptyTypes.
• Uncomment mapperWithEmptyT1 only.
  • Result: ❌ TypeScript error.
• Comment mapperWithEmptyT1 and uncomment mapperWithEmptyT2 only.
  • Result: ❌ TypeScript error.
• Uncomment both mapperWithEmptyT1 and mapperWithEmptyT2.
  • Result: ❌ TypeScript error.

2. Error does not occur when adding mapperWithBothTypes or mapperWithEmptyTypes

• Comment all mappers in MAPPERS.
• Uncomment mapperWithBothTypes, then uncomment any other combination of mappers (e.g., mapperWithEmptyT1, mapperWithEmptyT2).
  • Result: ✅ No TypeScript error, regardless of which other mappers are uncommented.
• Comment mapperWithBothTypes and instead uncomment mapperWithEmptyTypes.
  • Result: ✅ No TypeScript error, regardless of which other mappers are uncommented.

3. Workaround: Preventing the error using Partial<T1> or Partial<T2> without mapperWithBothTypes or mapperWithEmptyTypes

• Modify TypesMapper to change only mapT1 to Partial<T1>.
• Comment all mappers in MAPPERS, except for mapperWithEmptyT1.
  • Result: ❌ TypeScript error.
• Uncomment mapperWithEmptyT2, whether mapperWithEmptyT1 is uncommented or commented.
  • Result: ✅ No TypeScript error, because mapperWithEmptyT2 has a non-empty T1.
• Reverse the change: restore mapT1 and instead change mapT2 to Partial<T2>.
• Comment all mappers in MAPPERS, except for mapperWithEmptyT2.
  • Result: ❌ TypeScript error.
• Uncomment mapperWithEmptyT1 while keeping mapperWithEmptyT2 uncommented or commented.
  • Result: ✅ No TypeScript error, because mapperWithEmptyT1 has a non-empty T2.

Limitations of The Workaround

Using Partial<T1> or Partial<T2> as a workaround does not help when we want to enforce explicit converters for all fields. Since Partial<T> allows missing fields, it weakens type safety and fails to enforce strict mapping of all fields.

Additional Observation

This issue does not occur when using the same generic type with only one type parameter instead of two. In this case, TypeScript correctly infers and applies the expected type without any conversion errors.

Playground Link: a working example with only one generic type

🙂 Expected behavior

TypeScript should work consistently and always allow retrieving values from MAPPERS in the above generic function, without type conversion errors, regardless of the specific generic type combinations used.