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merged 5 commits into from
Mar 4, 2024
Merged

Add optionality to mapped type indexed access substitutions #57549

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97095c1
Add optionality to mapped type indexed access substitutions
ahejlsberg Feb 26, 2024
ab3184b
Add tests
ahejlsberg Feb 26, 2024
1de92c9
Examine modifiers types in depth
ahejlsberg Feb 28, 2024
e83df76
Add test
ahejlsberg Feb 28, 2024
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ahejlsberg Mar 4, 2024
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17 changes: 13 additions & 4 deletions src/compiler/checker.ts
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Expand Up @@ -14035,15 +14035,23 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
(declaration.questionToken ? declaration.questionToken.kind === SyntaxKind.MinusToken ? MappedTypeModifiers.ExcludeOptional : MappedTypeModifiers.IncludeOptional : 0);
}

// Return -1, 0, or 1, where -1 means optionality is stripped (i.e. -?), 0 means optionality is unchanged, and 1 means
// optionality is added (i.e. +?).
function getMappedTypeOptionality(type: MappedType): number {
const modifiers = getMappedTypeModifiers(type);
return modifiers & MappedTypeModifiers.ExcludeOptional ? -1 : modifiers & MappedTypeModifiers.IncludeOptional ? 1 : 0;
}
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I don't understand what this is computing. What does a result of 2 mean?

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The result is always either -1, 0, or 1. -1 means optionality is stripped (i.e. -?), 0 means optionality is unchanged, 1 means optionality is added (i.e. +?). When a homomorphic mapped type doesn't modify optionality, we (recursively) consult the optionality of the type you're mapping over to see if it strips or adds optionality.

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Should this be an enum? Then it'd be easier to tell what the numbers mean.

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It could be an enum, but number feels more appropriate since we use the greater and less than operators to relate the combined values.

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Would a comment about what each number represents work?


function getModifiersTypeOptionality(type: Type): number {
return type.flags & TypeFlags.Intersection ? Math.max(...map((type as IntersectionType).types, getModifiersTypeOptionality)) :
getObjectFlags(type) & ObjectFlags.Mapped ? getCombinedMappedTypeOptionality(type as MappedType) :
0;
}
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If I'm not mistaken this currently falls short here:

type Obj = {
    a: 1,
    b: 2
};

type Identity<T> = { [K in keyof T]: T[K] };

const mapped: {
    [K in keyof Identity<Partial<Obj>>]: Obj[K]
} = {}

const resolveMapped = <K extends keyof typeof mapped>(key: K) => mapped[key].toString(); // should error but it doesnt

Some extra handling of homomorphic mapped types might have to be added here.

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Yeah, looks like we need to recurse in getCombinedMappedTypeOptionality.


// Return -1, 0, or 1, for stripped, unchanged, or added optionality respectively. When a homomorphic mapped type doesn't
// modify optionality, recursively consult the optionality of the type being mapped over to see if it strips or adds optionality.
function getCombinedMappedTypeOptionality(type: MappedType): number {
const optionality = getMappedTypeOptionality(type);
const modifiersType = getModifiersTypeFromMappedType(type);
return optionality || (isGenericMappedType(modifiersType) ? getMappedTypeOptionality(modifiersType) : 0);
return getMappedTypeOptionality(type) || getModifiersTypeOptionality(getModifiersTypeFromMappedType(type));
}

function isPartialMappedType(type: Type) {
Expand Down Expand Up @@ -18517,7 +18525,8 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
function substituteIndexedMappedType(objectType: MappedType, index: Type) {
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Taking known properties into consideration would fix this inconsistency that this PR creates:

type Obj = {
  a: string;
  b: number;
};

type Obj2 = {
  b: number;
  c: boolean;
};

declare const mapped: {
  [K in keyof (Partial<Obj> & Required<Obj2>)]: number;
};

// displays the same way as `resolved` below!
mapped;
// ^? const mapped: { a?: number | undefined; b: number; c: number; }

const accessMapped = <K extends keyof Obj2>(key: K) => mapped[key].toString();

declare const resolved: { a?: number | undefined; b: number; c: number };

const accessResolved = <K extends keyof Obj2>(key: K) => resolved[key].toString();

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Right, when mapping over intersections constructed from multiple mapped types the PR makes the most conservative assumption about optionality. I think that's an acceptable compromise and I'm concerned with the potential cost of examining every member possibly selected by the key.

const mapper = createTypeMapper([getTypeParameterFromMappedType(objectType)], [index]);
const templateMapper = combineTypeMappers(objectType.mapper, mapper);
return instantiateType(getTemplateTypeFromMappedType(objectType.target as MappedType || objectType), templateMapper);
const instantiatedTemplateType = instantiateType(getTemplateTypeFromMappedType(objectType.target as MappedType || objectType), templateMapper);
return addOptionality(instantiatedTemplateType, /*isProperty*/ true, getCombinedMappedTypeOptionality(objectType) > 0);
}

function getIndexedAccessType(objectType: Type, indexType: Type, accessFlags = AccessFlags.None, accessNode?: ElementAccessExpression | IndexedAccessTypeNode | PropertyName | BindingName | SyntheticExpression, aliasSymbol?: Symbol, aliasTypeArguments?: readonly Type[]): Type {
Expand Down
75 changes: 75 additions & 0 deletions tests/baselines/reference/mappedTypeIndexedAccessConstraint.errors.txt
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@@ -0,0 +1,75 @@
mappedTypeIndexedAccessConstraint.ts(12,5): error TS18048: 'm1' is possibly 'undefined'.
mappedTypeIndexedAccessConstraint.ts(14,5): error TS18048: 'm2' is possibly 'undefined'.
mappedTypeIndexedAccessConstraint.ts(16,5): error TS18048: 'm3' is possibly 'undefined'.
mappedTypeIndexedAccessConstraint.ts(29,66): error TS2532: Object is possibly 'undefined'.
mappedTypeIndexedAccessConstraint.ts(53,34): error TS2722: Cannot invoke an object which is possibly 'undefined'.


==== mappedTypeIndexedAccessConstraint.ts (5 errors) ====
type Identity<T> = { [K in keyof T]: T[K] };

type M0 = { a: 1, b: 2 };

type M1 = { [K in keyof Partial<M0>]: M0[K] };

type M2 = { [K in keyof Required<M1>]: M1[K] };

type M3 = { [K in keyof Identity<Partial<M0>>]: M0[K] };

function foo<K extends keyof M0>(m1: M1[K], m2: M2[K], m3: M3[K]) {
m1.toString(); // Error
~~
!!! error TS18048: 'm1' is possibly 'undefined'.
m1?.toString();
m2.toString(); // Error
~~
!!! error TS18048: 'm2' is possibly 'undefined'.
m2?.toString();
m3.toString(); // Error
~~
!!! error TS18048: 'm3' is possibly 'undefined'.
m3?.toString();
}

// Repro from #57487

type Obj = {
a: 1,
b: 2
};

const mapped: { [K in keyof Partial<Obj>]: Obj[K] } = {};

const resolveMapped = <K extends keyof typeof mapped>(key: K) => mapped[key].toString(); // Error
~~~~~~~~~~~
!!! error TS2532: Object is possibly 'undefined'.

// Additional repro from #57487

const arr = ["foo", "12", 42] as const;

type Mappings = { foo: boolean, "12": number, 42: string };

type MapperArgs<K extends (typeof arr)[number]> = {
v: K,
i: number
};

type SetOptional<T, K extends keyof T> = Omit<T, K> & Partial<Pick<T, K>>;

type PartMappings = SetOptional<Mappings, "foo">;

const mapper: { [K in keyof PartMappings]: (o: MapperArgs<K>) => PartMappings[K] } = {
foo: ({ v, i }) => v.length + i > 4,
"12": ({ v, i }) => Number(v) + i,
42: ({ v, i }) => `${v}${i}`,
}

const resolveMapper1 = <K extends keyof typeof mapper>(
key: K, o: MapperArgs<K>) => mapper[key](o); // Error
~~~~~~~~~~~
!!! error TS2722: Cannot invoke an object which is possibly 'undefined'.

const resolveMapper2 = <K extends keyof typeof mapper>(
key: K, o: MapperArgs<K>) => mapper[key]?.(o)

227 changes: 227 additions & 0 deletions tests/baselines/reference/mappedTypeIndexedAccessConstraint.symbols
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@@ -0,0 +1,227 @@
//// [tests/cases/compiler/mappedTypeIndexedAccessConstraint.ts] ////

=== mappedTypeIndexedAccessConstraint.ts ===
type Identity<T> = { [K in keyof T]: T[K] };
>Identity : Symbol(Identity, Decl(mappedTypeIndexedAccessConstraint.ts, 0, 0))
>T : Symbol(T, Decl(mappedTypeIndexedAccessConstraint.ts, 0, 14))
>K : Symbol(K, Decl(mappedTypeIndexedAccessConstraint.ts, 0, 22))
>T : Symbol(T, Decl(mappedTypeIndexedAccessConstraint.ts, 0, 14))
>T : Symbol(T, Decl(mappedTypeIndexedAccessConstraint.ts, 0, 14))
>K : Symbol(K, Decl(mappedTypeIndexedAccessConstraint.ts, 0, 22))

type M0 = { a: 1, b: 2 };
>M0 : Symbol(M0, Decl(mappedTypeIndexedAccessConstraint.ts, 0, 44))
>a : Symbol(a, Decl(mappedTypeIndexedAccessConstraint.ts, 2, 11))
>b : Symbol(b, Decl(mappedTypeIndexedAccessConstraint.ts, 2, 17))

type M1 = { [K in keyof Partial<M0>]: M0[K] };
>M1 : Symbol(M1, Decl(mappedTypeIndexedAccessConstraint.ts, 2, 25))
>K : Symbol(K, Decl(mappedTypeIndexedAccessConstraint.ts, 4, 13))
>Partial : Symbol(Partial, Decl(lib.es5.d.ts, --, --))
>M0 : Symbol(M0, Decl(mappedTypeIndexedAccessConstraint.ts, 0, 44))
>M0 : Symbol(M0, Decl(mappedTypeIndexedAccessConstraint.ts, 0, 44))
>K : Symbol(K, Decl(mappedTypeIndexedAccessConstraint.ts, 4, 13))

type M2 = { [K in keyof Required<M1>]: M1[K] };
>M2 : Symbol(M2, Decl(mappedTypeIndexedAccessConstraint.ts, 4, 46))
>K : Symbol(K, Decl(mappedTypeIndexedAccessConstraint.ts, 6, 13))
>Required : Symbol(Required, Decl(lib.es5.d.ts, --, --))
>M1 : Symbol(M1, Decl(mappedTypeIndexedAccessConstraint.ts, 2, 25))
>M1 : Symbol(M1, Decl(mappedTypeIndexedAccessConstraint.ts, 2, 25))
>K : Symbol(K, Decl(mappedTypeIndexedAccessConstraint.ts, 6, 13))

type M3 = { [K in keyof Identity<Partial<M0>>]: M0[K] };
>M3 : Symbol(M3, Decl(mappedTypeIndexedAccessConstraint.ts, 6, 47))
>K : Symbol(K, Decl(mappedTypeIndexedAccessConstraint.ts, 8, 13))
>Identity : Symbol(Identity, Decl(mappedTypeIndexedAccessConstraint.ts, 0, 0))
>Partial : Symbol(Partial, Decl(lib.es5.d.ts, --, --))
>M0 : Symbol(M0, Decl(mappedTypeIndexedAccessConstraint.ts, 0, 44))
>M0 : Symbol(M0, Decl(mappedTypeIndexedAccessConstraint.ts, 0, 44))
>K : Symbol(K, Decl(mappedTypeIndexedAccessConstraint.ts, 8, 13))

function foo<K extends keyof M0>(m1: M1[K], m2: M2[K], m3: M3[K]) {
>foo : Symbol(foo, Decl(mappedTypeIndexedAccessConstraint.ts, 8, 56))
>K : Symbol(K, Decl(mappedTypeIndexedAccessConstraint.ts, 10, 13))
>M0 : Symbol(M0, Decl(mappedTypeIndexedAccessConstraint.ts, 0, 44))
>m1 : Symbol(m1, Decl(mappedTypeIndexedAccessConstraint.ts, 10, 33))
>M1 : Symbol(M1, Decl(mappedTypeIndexedAccessConstraint.ts, 2, 25))
>K : Symbol(K, Decl(mappedTypeIndexedAccessConstraint.ts, 10, 13))
>m2 : Symbol(m2, Decl(mappedTypeIndexedAccessConstraint.ts, 10, 43))
>M2 : Symbol(M2, Decl(mappedTypeIndexedAccessConstraint.ts, 4, 46))
>K : Symbol(K, Decl(mappedTypeIndexedAccessConstraint.ts, 10, 13))
>m3 : Symbol(m3, Decl(mappedTypeIndexedAccessConstraint.ts, 10, 54))
>M3 : Symbol(M3, Decl(mappedTypeIndexedAccessConstraint.ts, 6, 47))
>K : Symbol(K, Decl(mappedTypeIndexedAccessConstraint.ts, 10, 13))

m1.toString(); // Error
>m1.toString : Symbol(Number.toString, Decl(lib.es5.d.ts, --, --))
>m1 : Symbol(m1, Decl(mappedTypeIndexedAccessConstraint.ts, 10, 33))
>toString : Symbol(Number.toString, Decl(lib.es5.d.ts, --, --))

m1?.toString();
>m1?.toString : Symbol(Number.toString, Decl(lib.es5.d.ts, --, --))
>m1 : Symbol(m1, Decl(mappedTypeIndexedAccessConstraint.ts, 10, 33))
>toString : Symbol(Number.toString, Decl(lib.es5.d.ts, --, --))

m2.toString(); // Error
>m2.toString : Symbol(Number.toString, Decl(lib.es5.d.ts, --, --))
>m2 : Symbol(m2, Decl(mappedTypeIndexedAccessConstraint.ts, 10, 43))
>toString : Symbol(Number.toString, Decl(lib.es5.d.ts, --, --))

m2?.toString();
>m2?.toString : Symbol(Number.toString, Decl(lib.es5.d.ts, --, --))
>m2 : Symbol(m2, Decl(mappedTypeIndexedAccessConstraint.ts, 10, 43))
>toString : Symbol(Number.toString, Decl(lib.es5.d.ts, --, --))

m3.toString(); // Error
>m3.toString : Symbol(Number.toString, Decl(lib.es5.d.ts, --, --))
>m3 : Symbol(m3, Decl(mappedTypeIndexedAccessConstraint.ts, 10, 54))
>toString : Symbol(Number.toString, Decl(lib.es5.d.ts, --, --))

m3?.toString();
>m3?.toString : Symbol(Number.toString, Decl(lib.es5.d.ts, --, --))
>m3 : Symbol(m3, Decl(mappedTypeIndexedAccessConstraint.ts, 10, 54))
>toString : Symbol(Number.toString, Decl(lib.es5.d.ts, --, --))
}

// Repro from #57487

type Obj = {
>Obj : Symbol(Obj, Decl(mappedTypeIndexedAccessConstraint.ts, 17, 1))

a: 1,
>a : Symbol(a, Decl(mappedTypeIndexedAccessConstraint.ts, 21, 12))

b: 2
>b : Symbol(b, Decl(mappedTypeIndexedAccessConstraint.ts, 22, 9))

};

const mapped: { [K in keyof Partial<Obj>]: Obj[K] } = {};
>mapped : Symbol(mapped, Decl(mappedTypeIndexedAccessConstraint.ts, 26, 5))
>K : Symbol(K, Decl(mappedTypeIndexedAccessConstraint.ts, 26, 17))
>Partial : Symbol(Partial, Decl(lib.es5.d.ts, --, --))
>Obj : Symbol(Obj, Decl(mappedTypeIndexedAccessConstraint.ts, 17, 1))
>Obj : Symbol(Obj, Decl(mappedTypeIndexedAccessConstraint.ts, 17, 1))
>K : Symbol(K, Decl(mappedTypeIndexedAccessConstraint.ts, 26, 17))

const resolveMapped = <K extends keyof typeof mapped>(key: K) => mapped[key].toString(); // Error
>resolveMapped : Symbol(resolveMapped, Decl(mappedTypeIndexedAccessConstraint.ts, 28, 5))
>K : Symbol(K, Decl(mappedTypeIndexedAccessConstraint.ts, 28, 23))
>mapped : Symbol(mapped, Decl(mappedTypeIndexedAccessConstraint.ts, 26, 5))
>key : Symbol(key, Decl(mappedTypeIndexedAccessConstraint.ts, 28, 54))
>K : Symbol(K, Decl(mappedTypeIndexedAccessConstraint.ts, 28, 23))
>mapped[key].toString : Symbol(Number.toString, Decl(lib.es5.d.ts, --, --))
>mapped : Symbol(mapped, Decl(mappedTypeIndexedAccessConstraint.ts, 26, 5))
>key : Symbol(key, Decl(mappedTypeIndexedAccessConstraint.ts, 28, 54))
>toString : Symbol(Number.toString, Decl(lib.es5.d.ts, --, --))

// Additional repro from #57487

const arr = ["foo", "12", 42] as const;
>arr : Symbol(arr, Decl(mappedTypeIndexedAccessConstraint.ts, 32, 5))
>const : Symbol(const)

type Mappings = { foo: boolean, "12": number, 42: string };
>Mappings : Symbol(Mappings, Decl(mappedTypeIndexedAccessConstraint.ts, 32, 39))
>foo : Symbol(foo, Decl(mappedTypeIndexedAccessConstraint.ts, 34, 17))
>"12" : Symbol("12", Decl(mappedTypeIndexedAccessConstraint.ts, 34, 31))
>42 : Symbol(42, Decl(mappedTypeIndexedAccessConstraint.ts, 34, 45))

type MapperArgs<K extends (typeof arr)[number]> = {
>MapperArgs : Symbol(MapperArgs, Decl(mappedTypeIndexedAccessConstraint.ts, 34, 59))
>K : Symbol(K, Decl(mappedTypeIndexedAccessConstraint.ts, 36, 16))
>arr : Symbol(arr, Decl(mappedTypeIndexedAccessConstraint.ts, 32, 5))

v: K,
>v : Symbol(v, Decl(mappedTypeIndexedAccessConstraint.ts, 36, 51))
>K : Symbol(K, Decl(mappedTypeIndexedAccessConstraint.ts, 36, 16))

i: number
>i : Symbol(i, Decl(mappedTypeIndexedAccessConstraint.ts, 37, 9))

};

type SetOptional<T, K extends keyof T> = Omit<T, K> & Partial<Pick<T, K>>;
>SetOptional : Symbol(SetOptional, Decl(mappedTypeIndexedAccessConstraint.ts, 39, 2))
>T : Symbol(T, Decl(mappedTypeIndexedAccessConstraint.ts, 41, 17))
>K : Symbol(K, Decl(mappedTypeIndexedAccessConstraint.ts, 41, 19))
>T : Symbol(T, Decl(mappedTypeIndexedAccessConstraint.ts, 41, 17))
>Omit : Symbol(Omit, Decl(lib.es5.d.ts, --, --))
>T : Symbol(T, Decl(mappedTypeIndexedAccessConstraint.ts, 41, 17))
>K : Symbol(K, Decl(mappedTypeIndexedAccessConstraint.ts, 41, 19))
>Partial : Symbol(Partial, Decl(lib.es5.d.ts, --, --))
>Pick : Symbol(Pick, Decl(lib.es5.d.ts, --, --))
>T : Symbol(T, Decl(mappedTypeIndexedAccessConstraint.ts, 41, 17))
>K : Symbol(K, Decl(mappedTypeIndexedAccessConstraint.ts, 41, 19))

type PartMappings = SetOptional<Mappings, "foo">;
>PartMappings : Symbol(PartMappings, Decl(mappedTypeIndexedAccessConstraint.ts, 41, 74))
>SetOptional : Symbol(SetOptional, Decl(mappedTypeIndexedAccessConstraint.ts, 39, 2))
>Mappings : Symbol(Mappings, Decl(mappedTypeIndexedAccessConstraint.ts, 32, 39))

const mapper: { [K in keyof PartMappings]: (o: MapperArgs<K>) => PartMappings[K] } = {
>mapper : Symbol(mapper, Decl(mappedTypeIndexedAccessConstraint.ts, 45, 5))
>K : Symbol(K, Decl(mappedTypeIndexedAccessConstraint.ts, 45, 17))
>PartMappings : Symbol(PartMappings, Decl(mappedTypeIndexedAccessConstraint.ts, 41, 74))
>o : Symbol(o, Decl(mappedTypeIndexedAccessConstraint.ts, 45, 44))
>MapperArgs : Symbol(MapperArgs, Decl(mappedTypeIndexedAccessConstraint.ts, 34, 59))
>K : Symbol(K, Decl(mappedTypeIndexedAccessConstraint.ts, 45, 17))
>PartMappings : Symbol(PartMappings, Decl(mappedTypeIndexedAccessConstraint.ts, 41, 74))
>K : Symbol(K, Decl(mappedTypeIndexedAccessConstraint.ts, 45, 17))

foo: ({ v, i }) => v.length + i > 4,
>foo : Symbol(foo, Decl(mappedTypeIndexedAccessConstraint.ts, 45, 86))
>v : Symbol(v, Decl(mappedTypeIndexedAccessConstraint.ts, 46, 11))
>i : Symbol(i, Decl(mappedTypeIndexedAccessConstraint.ts, 46, 14))
>v.length : Symbol(String.length, Decl(lib.es5.d.ts, --, --))
>v : Symbol(v, Decl(mappedTypeIndexedAccessConstraint.ts, 46, 11))
>length : Symbol(String.length, Decl(lib.es5.d.ts, --, --))
>i : Symbol(i, Decl(mappedTypeIndexedAccessConstraint.ts, 46, 14))

"12": ({ v, i }) => Number(v) + i,
>"12" : Symbol("12", Decl(mappedTypeIndexedAccessConstraint.ts, 46, 40))
>v : Symbol(v, Decl(mappedTypeIndexedAccessConstraint.ts, 47, 12))
>i : Symbol(i, Decl(mappedTypeIndexedAccessConstraint.ts, 47, 15))
>Number : Symbol(Number, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
>v : Symbol(v, Decl(mappedTypeIndexedAccessConstraint.ts, 47, 12))
>i : Symbol(i, Decl(mappedTypeIndexedAccessConstraint.ts, 47, 15))

42: ({ v, i }) => `${v}${i}`,
>42 : Symbol(42, Decl(mappedTypeIndexedAccessConstraint.ts, 47, 38))
>v : Symbol(v, Decl(mappedTypeIndexedAccessConstraint.ts, 48, 10))
>i : Symbol(i, Decl(mappedTypeIndexedAccessConstraint.ts, 48, 13))
>v : Symbol(v, Decl(mappedTypeIndexedAccessConstraint.ts, 48, 10))
>i : Symbol(i, Decl(mappedTypeIndexedAccessConstraint.ts, 48, 13))
}

const resolveMapper1 = <K extends keyof typeof mapper>(
>resolveMapper1 : Symbol(resolveMapper1, Decl(mappedTypeIndexedAccessConstraint.ts, 51, 5))
>K : Symbol(K, Decl(mappedTypeIndexedAccessConstraint.ts, 51, 24))
>mapper : Symbol(mapper, Decl(mappedTypeIndexedAccessConstraint.ts, 45, 5))

key: K, o: MapperArgs<K>) => mapper[key](o); // Error
>key : Symbol(key, Decl(mappedTypeIndexedAccessConstraint.ts, 51, 55))
>K : Symbol(K, Decl(mappedTypeIndexedAccessConstraint.ts, 51, 24))
>o : Symbol(o, Decl(mappedTypeIndexedAccessConstraint.ts, 52, 11))
>MapperArgs : Symbol(MapperArgs, Decl(mappedTypeIndexedAccessConstraint.ts, 34, 59))
>K : Symbol(K, Decl(mappedTypeIndexedAccessConstraint.ts, 51, 24))
>mapper : Symbol(mapper, Decl(mappedTypeIndexedAccessConstraint.ts, 45, 5))
>key : Symbol(key, Decl(mappedTypeIndexedAccessConstraint.ts, 51, 55))
>o : Symbol(o, Decl(mappedTypeIndexedAccessConstraint.ts, 52, 11))

const resolveMapper2 = <K extends keyof typeof mapper>(
>resolveMapper2 : Symbol(resolveMapper2, Decl(mappedTypeIndexedAccessConstraint.ts, 54, 5))
>K : Symbol(K, Decl(mappedTypeIndexedAccessConstraint.ts, 54, 24))
>mapper : Symbol(mapper, Decl(mappedTypeIndexedAccessConstraint.ts, 45, 5))

key: K, o: MapperArgs<K>) => mapper[key]?.(o)
>key : Symbol(key, Decl(mappedTypeIndexedAccessConstraint.ts, 54, 55))
>K : Symbol(K, Decl(mappedTypeIndexedAccessConstraint.ts, 54, 24))
>o : Symbol(o, Decl(mappedTypeIndexedAccessConstraint.ts, 55, 11))
>MapperArgs : Symbol(MapperArgs, Decl(mappedTypeIndexedAccessConstraint.ts, 34, 59))
>K : Symbol(K, Decl(mappedTypeIndexedAccessConstraint.ts, 54, 24))
>mapper : Symbol(mapper, Decl(mappedTypeIndexedAccessConstraint.ts, 45, 5))
>key : Symbol(key, Decl(mappedTypeIndexedAccessConstraint.ts, 54, 55))
>o : Symbol(o, Decl(mappedTypeIndexedAccessConstraint.ts, 55, 11))

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