Distribute index over mapped types when normalizing types

Author: Andarist

src/compiler/checker.ts

@@ -17562,8 +17562,8 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
             (type.flags & (TypeFlags.InstantiableNonPrimitive | TypeFlags.Index | TypeFlags.StringMapping) && !isPatternLiteralType(type) ? ObjectFlags.IsGenericIndexType : 0);
     }
 
-    function getSimplifiedType(type: Type, writing: boolean): Type {
-        return type.flags & TypeFlags.IndexedAccess ? getSimplifiedIndexedAccessType(type as IndexedAccessType, writing) :
+    function getSimplifiedType(type: Type, writing: boolean, distributeIndexOverMappedType = false): Type {
+        return type.flags & TypeFlags.IndexedAccess ? getSimplifiedIndexedAccessType(type as IndexedAccessType, writing, distributeIndexOverMappedType) :
             type.flags & TypeFlags.Conditional ? getSimplifiedConditionalType(type as ConditionalType, writing) :
             type;
     }
@@ -17590,8 +17590,8 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
     // Transform an indexed access to a simpler form, if possible. Return the simpler form, or return
     // the type itself if no transformation is possible. The writing flag indicates that the type is
     // the target of an assignment.
-    function getSimplifiedIndexedAccessType(type: IndexedAccessType, writing: boolean): Type {
-        const cache = writing ? "simplifiedForWriting" : "simplifiedForReading";
+    function getSimplifiedIndexedAccessType(type: IndexedAccessType, writing: boolean, distributeIndexOverMappedType: boolean): Type {
+        const cache = writing ? "simplifiedForWriting" : `simplifiedForReading${distributeIndexOverMappedType ? "D" : ""}` as const;
         if (type[cache]) {
             return type[cache] === circularConstraintType ? type : type[cache]!;
         }
@@ -17633,6 +17633,13 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
         // For example, for an index access { [P in K]: Box<T[P]> }[X], we construct the type Box<T[X]>.
         if (isGenericMappedType(objectType)) {
             if (!getNameTypeFromMappedType(objectType) || isFilteringMappedType(objectType)) {
+                if (distributeIndexOverMappedType && !writing) {
+                    const indexConstraint = indexType.flags & TypeFlags.Instantiable && getBaseConstraintOfType(indexType);
+                    if (indexConstraint && indexConstraint.flags & TypeFlags.Union) {
+                        return type[cache] = getSimplifiedType(getIndexedAccessType(objectType, getIntersectionType([indexType, indexConstraint])), writing);
+                    }
+                }
+
                 return type[cache] = mapType(substituteIndexedMappedType(objectType, type.indexType), t => getSimplifiedType(t, writing));
             }
         }
@@ -20263,7 +20270,7 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
                 getObjectFlags(type) & ObjectFlags.Reference ? (type as TypeReference).node ? createTypeReference((type as TypeReference).target, getTypeArguments(type as TypeReference)) : getSingleBaseForNonAugmentingSubtype(type) || type :
                 type.flags & TypeFlags.UnionOrIntersection ? getNormalizedUnionOrIntersectionType(type as UnionOrIntersectionType, writing) :
                 type.flags & TypeFlags.Substitution ? writing ? (type as SubstitutionType).baseType : getSubstitutionIntersection(type as SubstitutionType) :
-                type.flags & TypeFlags.Simplifiable ? getSimplifiedType(type, writing) :
+                type.flags & TypeFlags.Simplifiable ? getSimplifiedType(type, writing, /*distributeIndexOverMappedType*/ true) :
                 type;
             if (t === type) return t;
             type = t;

src/compiler/types.ts

@@ -6624,6 +6624,7 @@ export interface IndexedAccessType extends InstantiableType {
     constraint?: Type;
     simplifiedForReading?: Type;
     simplifiedForWriting?: Type;
+    simplifiedForReadingD?: Type;
 }
 
 export type TypeVariable = TypeParameter | IndexedAccessType;

tests/baselines/reference/api/tsserverlibrary.d.ts

@@ -6983,6 +6983,7 @@ declare namespace ts {
         constraint?: Type;
         simplifiedForReading?: Type;
         simplifiedForWriting?: Type;
+        simplifiedForReadingD?: Type;
     }
     type TypeVariable = TypeParameter | IndexedAccessType;
     interface IndexType extends InstantiableType {

tests/baselines/reference/api/typescript.d.ts

@@ -2925,6 +2925,7 @@ declare namespace ts {
         constraint?: Type;
         simplifiedForReading?: Type;
         simplifiedForWriting?: Type;
+        simplifiedForReadingD?: Type;
     }
     type TypeVariable = TypeParameter | IndexedAccessType;
     interface IndexType extends InstantiableType {

tests/baselines/reference/conditionalTypes1.errors.txt

@@ -8,7 +8,7 @@ conditionalTypes1.ts(17,5): error TS2322: Type 'T' is not assignable to type 'No
           Type 'T' is not assignable to type '{}'.
             Type 'string | undefined' is not assignable to type '{}'.
               Type 'undefined' is not assignable to type '{}'.
-conditionalTypes1.ts(24,5): error TS2322: Type 'T[keyof T] | undefined' is not assignable to type 'NonNullable<Partial<T>[keyof T]>'.
+conditionalTypes1.ts(24,5): error TS2322: Type 'T[keyof T & string] | T[keyof T & number] | T[keyof T & symbol] | undefined' is not assignable to type 'NonNullable<Partial<T>[keyof T]>'.
   Type 'undefined' is not assignable to type 'T[keyof T] & {}'.
     Type 'undefined' is not assignable to type 'T[keyof T]'.
       'T[keyof T]' could be instantiated with an arbitrary type which could be unrelated to 'undefined'.
@@ -25,32 +25,28 @@ conditionalTypes1.ts(104,5): error TS2322: Type 'NonFunctionProperties<T>' is no
   'T' could be instantiated with an arbitrary type which could be unrelated to 'NonFunctionProperties<T>'.
 conditionalTypes1.ts(106,5): error TS2322: Type 'NonFunctionProperties<T>' is not assignable to type 'FunctionProperties<T>'.
   Type 'FunctionPropertyNames<T>' is not assignable to type 'NonFunctionPropertyNames<T>'.
-    Type 'keyof T' is not assignable to type 'T[keyof T] extends Function ? never : keyof T'.
-      Type 'string | number | symbol' is not assignable to type 'T[keyof T] extends Function ? never : keyof T'.
-        Type 'string' is not assignable to type 'T[keyof T] extends Function ? never : keyof T'.
-          Type 'keyof T' is not assignable to type 'never'.
-            Type 'string | number | symbol' is not assignable to type 'never'.
-              Type 'string' is not assignable to type 'never'.
+    Type 'T[keyof T & string] extends Function ? keyof T & string : never' is not assignable to type 'T[keyof T] extends Function ? never : keyof T'.
+      Type 'keyof T & string' is not assignable to type 'T[keyof T] extends Function ? never : keyof T'.
+        Type 'keyof T & string' is not assignable to type 'never'.
 conditionalTypes1.ts(108,5): error TS2322: Type 'FunctionProperties<T>' is not assignable to type 'NonFunctionProperties<T>'.
   Type 'NonFunctionPropertyNames<T>' is not assignable to type 'FunctionPropertyNames<T>'.
-    Type 'keyof T' is not assignable to type 'T[keyof T] extends Function ? keyof T : never'.
-      Type 'string | number | symbol' is not assignable to type 'T[keyof T] extends Function ? keyof T : never'.
-        Type 'string' is not assignable to type 'T[keyof T] extends Function ? keyof T : never'.
-          Type 'keyof T' is not assignable to type 'never'.
+    Type 'T[keyof T & string] extends Function ? never : keyof T & string' is not assignable to type 'T[keyof T] extends Function ? keyof T : never'.
+      Type 'keyof T & string' is not assignable to type 'T[keyof T] extends Function ? keyof T : never'.
+        Type 'keyof T & string' is not assignable to type 'never'.
 conditionalTypes1.ts(114,5): error TS2322: Type 'keyof T' is not assignable to type 'FunctionPropertyNames<T>'.
   Type 'string | number | symbol' is not assignable to type 'T[keyof T] extends Function ? keyof T : never'.
     Type 'string' is not assignable to type 'T[keyof T] extends Function ? keyof T : never'.
 conditionalTypes1.ts(115,5): error TS2322: Type 'NonFunctionPropertyNames<T>' is not assignable to type 'FunctionPropertyNames<T>'.
-  Type 'keyof T' is not assignable to type 'T[keyof T] extends Function ? keyof T : never'.
-    Type 'keyof T' is not assignable to type 'never'.
-      Type 'string | number | symbol' is not assignable to type 'never'.
-        Type 'string' is not assignable to type 'never'.
+  Type 'T[keyof T & string] extends Function ? never : keyof T & string' is not assignable to type 'T[keyof T] extends Function ? keyof T : never'.
+    Type 'keyof T & string' is not assignable to type 'T[keyof T] extends Function ? keyof T : never'.
+      Type 'keyof T & string' is not assignable to type 'never'.
 conditionalTypes1.ts(116,5): error TS2322: Type 'keyof T' is not assignable to type 'NonFunctionPropertyNames<T>'.
   Type 'string | number | symbol' is not assignable to type 'T[keyof T] extends Function ? never : keyof T'.
     Type 'string' is not assignable to type 'T[keyof T] extends Function ? never : keyof T'.
 conditionalTypes1.ts(117,5): error TS2322: Type 'FunctionPropertyNames<T>' is not assignable to type 'NonFunctionPropertyNames<T>'.
-  Type 'keyof T' is not assignable to type 'T[keyof T] extends Function ? never : keyof T'.
-    Type 'keyof T' is not assignable to type 'never'.
+  Type 'T[keyof T & string] extends Function ? keyof T & string : never' is not assignable to type 'T[keyof T] extends Function ? never : keyof T'.
+    Type 'keyof T & string' is not assignable to type 'T[keyof T] extends Function ? never : keyof T'.
+      Type 'keyof T & string' is not assignable to type 'never'.
 conditionalTypes1.ts(134,10): error TS2540: Cannot assign to 'id' because it is a read-only property.
 conditionalTypes1.ts(135,5): error TS2542: Index signature in type 'DeepReadonlyArray<Part>' only permits reading.
 conditionalTypes1.ts(136,22): error TS2540: Cannot assign to 'id' because it is a read-only property.
@@ -108,7 +104,7 @@ conditionalTypes1.ts(288,43): error TS2322: Type 'T95<U>' is not assignable to t
         x = y;
         y = x;  // Error
         ~
-!!! error TS2322: Type 'T[keyof T] | undefined' is not assignable to type 'NonNullable<Partial<T>[keyof T]>'.
+!!! error TS2322: Type 'T[keyof T & string] | T[keyof T & number] | T[keyof T & symbol] | undefined' is not assignable to type 'NonNullable<Partial<T>[keyof T]>'.
 !!! error TS2322:   Type 'undefined' is not assignable to type 'T[keyof T] & {}'.
 !!! error TS2322:     Type 'undefined' is not assignable to type 'T[keyof T]'.
 !!! error TS2322:       'T[keyof T]' could be instantiated with an arbitrary type which could be unrelated to 'undefined'.
@@ -211,21 +207,17 @@ conditionalTypes1.ts(288,43): error TS2322: Type 'T95<U>' is not assignable to t
         ~
 !!! error TS2322: Type 'NonFunctionProperties<T>' is not assignable to type 'FunctionProperties<T>'.
 !!! error TS2322:   Type 'FunctionPropertyNames<T>' is not assignable to type 'NonFunctionPropertyNames<T>'.
-!!! error TS2322:     Type 'keyof T' is not assignable to type 'T[keyof T] extends Function ? never : keyof T'.
-!!! error TS2322:       Type 'string | number | symbol' is not assignable to type 'T[keyof T] extends Function ? never : keyof T'.
-!!! error TS2322:         Type 'string' is not assignable to type 'T[keyof T] extends Function ? never : keyof T'.
-!!! error TS2322:           Type 'keyof T' is not assignable to type 'never'.
-!!! error TS2322:             Type 'string | number | symbol' is not assignable to type 'never'.
-!!! error TS2322:               Type 'string' is not assignable to type 'never'.
+!!! error TS2322:     Type 'T[keyof T & string] extends Function ? keyof T & string : never' is not assignable to type 'T[keyof T] extends Function ? never : keyof T'.
+!!! error TS2322:       Type 'keyof T & string' is not assignable to type 'T[keyof T] extends Function ? never : keyof T'.
+!!! error TS2322:         Type 'keyof T & string' is not assignable to type 'never'.
         z = x;
         z = y;  // Error
         ~
 !!! error TS2322: Type 'FunctionProperties<T>' is not assignable to type 'NonFunctionProperties<T>'.
 !!! error TS2322:   Type 'NonFunctionPropertyNames<T>' is not assignable to type 'FunctionPropertyNames<T>'.
-!!! error TS2322:     Type 'keyof T' is not assignable to type 'T[keyof T] extends Function ? keyof T : never'.
-!!! error TS2322:       Type 'string | number | symbol' is not assignable to type 'T[keyof T] extends Function ? keyof T : never'.
-!!! error TS2322:         Type 'string' is not assignable to type 'T[keyof T] extends Function ? keyof T : never'.
-!!! error TS2322:           Type 'keyof T' is not assignable to type 'never'.
+!!! error TS2322:     Type 'T[keyof T & string] extends Function ? never : keyof T & string' is not assignable to type 'T[keyof T] extends Function ? keyof T : never'.
+!!! error TS2322:       Type 'keyof T & string' is not assignable to type 'T[keyof T] extends Function ? keyof T : never'.
+!!! error TS2322:         Type 'keyof T & string' is not assignable to type 'never'.
     }
 
     function f8<T>(x: keyof T, y: FunctionPropertyNames<T>, z: NonFunctionPropertyNames<T>) {
@@ -239,10 +231,9 @@ conditionalTypes1.ts(288,43): error TS2322: Type 'T95<U>' is not assignable to t
         y = z;  // Error
         ~
 !!! error TS2322: Type 'NonFunctionPropertyNames<T>' is not assignable to type 'FunctionPropertyNames<T>'.
-!!! error TS2322:   Type 'keyof T' is not assignable to type 'T[keyof T] extends Function ? keyof T : never'.
-!!! error TS2322:     Type 'keyof T' is not assignable to type 'never'.
-!!! error TS2322:       Type 'string | number | symbol' is not assignable to type 'never'.
-!!! error TS2322:         Type 'string' is not assignable to type 'never'.
+!!! error TS2322:   Type 'T[keyof T & string] extends Function ? never : keyof T & string' is not assignable to type 'T[keyof T] extends Function ? keyof T : never'.
+!!! error TS2322:     Type 'keyof T & string' is not assignable to type 'T[keyof T] extends Function ? keyof T : never'.
+!!! error TS2322:       Type 'keyof T & string' is not assignable to type 'never'.
         z = x;  // Error
         ~
 !!! error TS2322: Type 'keyof T' is not assignable to type 'NonFunctionPropertyNames<T>'.
@@ -251,8 +242,9 @@ conditionalTypes1.ts(288,43): error TS2322: Type 'T95<U>' is not assignable to t
         z = y;  // Error
         ~
 !!! error TS2322: Type 'FunctionPropertyNames<T>' is not assignable to type 'NonFunctionPropertyNames<T>'.
-!!! error TS2322:   Type 'keyof T' is not assignable to type 'T[keyof T] extends Function ? never : keyof T'.
-!!! error TS2322:     Type 'keyof T' is not assignable to type 'never'.
+!!! error TS2322:   Type 'T[keyof T & string] extends Function ? keyof T & string : never' is not assignable to type 'T[keyof T] extends Function ? never : keyof T'.
+!!! error TS2322:     Type 'keyof T & string' is not assignable to type 'T[keyof T] extends Function ? never : keyof T'.
+!!! error TS2322:       Type 'keyof T & string' is not assignable to type 'never'.
     }
 
     type DeepReadonly<T> =