Defer types like keyof (T & {})

src/compiler/checker.ts

@@ -15238,20 +15238,24 @@ namespace ts {
          * type parameters in the union with a special never type that is treated as a literal in `getReducedType`, we can cause the `getReducedType` logic
          * to reduce the resulting type if possible (since only intersections with conflicting literal-typed properties are reducible).
          */
-        function isPossiblyReducibleByInstantiation(type: UnionType): boolean {
-            return some(type.types, t => {
-                const uniqueFilled = getUniqueLiteralFilledInstantiation(t);
-                return getReducedType(uniqueFilled) !== uniqueFilled;
-            });
+        function isPossiblyReducibleByInstantiation(type: Type): boolean {
+            const uniqueFilled = getUniqueLiteralFilledInstantiation(type);
+            return getReducedType(uniqueFilled) !== uniqueFilled;
+        }
+
+        function shouldDeferIndexType(type: Type) {
+            return !!(type.flags & TypeFlags.InstantiableNonPrimitive ||
+                isGenericTupleType(type) ||
+                isGenericMappedType(type) && !hasDistributiveNameType(type) ||
+                type.flags & TypeFlags.Union && some((type as UnionType).types, isPossiblyReducibleByInstantiation) ||
+                type.flags & TypeFlags.Intersection && maybeTypeOfKind(type, TypeFlags.Instantiable) && some((type as IntersectionType).types, isEmptyAnonymousObjectType));
         }
 
         function getIndexType(type: Type, stringsOnly = keyofStringsOnly, noIndexSignatures?: boolean): Type {
             type = getReducedType(type);
-            return type.flags & TypeFlags.Union ? isPossiblyReducibleByInstantiation(type as UnionType)
-                    ? getIndexTypeForGenericType(type as InstantiableType | UnionOrIntersectionType, stringsOnly)
-                    : getIntersectionType(map((type as UnionType).types, t => getIndexType(t, stringsOnly, noIndexSignatures))) :
+            return shouldDeferIndexType(type) ? getIndexTypeForGenericType(type as InstantiableType | UnionOrIntersectionType, stringsOnly) :
+                type.flags & TypeFlags.Union ? getIntersectionType(map((type as UnionType).types, t => getIndexType(t, stringsOnly, noIndexSignatures))) :
                 type.flags & TypeFlags.Intersection ? getUnionType(map((type as IntersectionType).types, t => getIndexType(t, stringsOnly, noIndexSignatures))) :
-                type.flags & TypeFlags.InstantiableNonPrimitive || isGenericTupleType(type) || isGenericMappedType(type) && !hasDistributiveNameType(type) ? getIndexTypeForGenericType(type as InstantiableType | UnionOrIntersectionType, stringsOnly) :
                 getObjectFlags(type) & ObjectFlags.Mapped ? getIndexTypeForMappedType(type as MappedType, stringsOnly, noIndexSignatures) :
                 type === wildcardType ? wildcardType :
                 type.flags & TypeFlags.Unknown ? neverType :

tests/baselines/reference/mappedTypes4.js

@@ -63,7 +63,7 @@ var x1: DeepReadonlyFoo;
 
 type Z = { a: number };
 type Clone<T> = {
-  [P in keyof (T & {})]: T[P];
+  [P in keyof (T & {})]: (T & {})[P];
 };
 type M = Clone<Z>; // M should be { a: number }
 
@@ -144,7 +144,7 @@ declare type Z = {
     a: number;
 };
 declare type Clone<T> = {
-    [P in keyof (T & {})]: T[P];
+    [P in keyof (T & {})]: (T & {})[P];
 };
 declare type M = Clone<Z>;
 declare var z1: Z;

tests/baselines/reference/mappedTypes4.symbols

@@ -205,7 +205,7 @@ type Clone<T> = {
 >Clone : Symbol(Clone, Decl(mappedTypes4.ts, 62, 23))
 >T : Symbol(T, Decl(mappedTypes4.ts, 63, 11))
 
-  [P in keyof (T & {})]: T[P];
+  [P in keyof (T & {})]: (T & {})[P];
 >P : Symbol(P, Decl(mappedTypes4.ts, 64, 3))
 >T : Symbol(T, Decl(mappedTypes4.ts, 63, 11))
 >T : Symbol(T, Decl(mappedTypes4.ts, 63, 11))

tests/baselines/reference/mappedTypes4.types

@@ -160,10 +160,10 @@ type Z = { a: number };
 type Clone<T> = {
 >Clone : Clone<T>
 
-  [P in keyof (T & {})]: T[P];
+  [P in keyof (T & {})]: (T & {})[P];
 };
 type M = Clone<Z>; // M should be { a: number }
->M : Clone<Z>
+>M : { a: number; }
 
 var z1: Z;
 >z1 : Z

tests/baselines/reference/unknownControlFlow.errors.txt

@@ -1,7 +1,11 @@
 tests/cases/conformance/types/unknown/unknownControlFlow.ts(18,9): error TS2322: Type 'unknown' is not assignable to type '{}'.
+tests/cases/conformance/types/unknown/unknownControlFlow.ts(283,5): error TS2536: Type 'keyof (T & {})' cannot be used to index type 'T'.
+tests/cases/conformance/types/unknown/unknownControlFlow.ts(290,11): error TS2345: Argument of type 'string' is not assignable to parameter of type 'never'.
+tests/cases/conformance/types/unknown/unknownControlFlow.ts(291,5): error TS2345: Argument of type 'null' is not assignable to parameter of type 'never'.
+tests/cases/conformance/types/unknown/unknownControlFlow.ts(293,5): error TS2345: Argument of type 'null' is not assignable to parameter of type 'never'.
 
 
-==== tests/cases/conformance/types/unknown/unknownControlFlow.ts (1 errors) ====
+==== tests/cases/conformance/types/unknown/unknownControlFlow.ts (5 errors) ====
     type T01 = {} & string;  // string
     type T02 = {} & 'a';  // 'a'
     type T03 = {} & object;  // object
@@ -272,4 +276,44 @@ tests/cases/conformance/types/unknown/unknownControlFlow.ts(18,9): error TS2322:
             y;
         }
     }
+
+    // We allow an unconstrained object of a generic type `T` to be indexed by a key of type `keyof T`
+    // without a check that the object is non-undefined and non-null. This is safe because `keyof T`
+    // is `never` (meaning no possible keys) for any `T` that includes `undefined` or `null`.
+
+    function ff1<T>(t: T, k: keyof T) {
+        t[k];
+    }
+
+    function ff2<T>(t: T & {}, k: keyof T) {
+        t[k];
+    }
+
+    function ff3<T>(t: T, k: keyof (T & {})) {
+        t[k];  // Error
+        ~~~~
+!!! error TS2536: Type 'keyof (T & {})' cannot be used to index type 'T'.
+    }
+
+    function ff4<T>(t: T & {}, k: keyof (T & {})) {
+        t[k];
+    }
+
+    ff1(null, 'foo');  // Error
+              ~~~~~
+!!! error TS2345: Argument of type 'string' is not assignable to parameter of type 'never'.
+    ff2(null, 'foo');  // Error
+        ~~~~
+!!! error TS2345: Argument of type 'null' is not assignable to parameter of type 'never'.
+    ff3(null, 'foo');
+    ff4(null, 'foo');  // Error
+        ~~~~
+!!! error TS2345: Argument of type 'null' is not assignable to parameter of type 'never'.
+
+    // Repro from #49681
+
+    type Foo = { [key: string]: unknown };
+    type NullableFoo = Foo | undefined;
+
+    type Bar<T extends NullableFoo> = NonNullable<T>[string];
 

tests/baselines/reference/unknownControlFlow.js

@@ -267,6 +267,38 @@ function foo<T>(x: T | null) {
         y;
     }
 }
+
+// We allow an unconstrained object of a generic type `T` to be indexed by a key of type `keyof T`
+// without a check that the object is non-undefined and non-null. This is safe because `keyof T`
+// is `never` (meaning no possible keys) for any `T` that includes `undefined` or `null`.
+
+function ff1<T>(t: T, k: keyof T) {
+    t[k];
+}
+
+function ff2<T>(t: T & {}, k: keyof T) {
+    t[k];
+}
+
+function ff3<T>(t: T, k: keyof (T & {})) {
+    t[k];  // Error
+}
+
+function ff4<T>(t: T & {}, k: keyof (T & {})) {
+    t[k];
+}
+
+ff1(null, 'foo');  // Error
+ff2(null, 'foo');  // Error
+ff3(null, 'foo');
+ff4(null, 'foo');  // Error
+
+// Repro from #49681
+
+type Foo = { [key: string]: unknown };
+type NullableFoo = Foo | undefined;
+
+type Bar<T extends NullableFoo> = NonNullable<T>[string];
 
 
 //// [unknownControlFlow.js]
@@ -501,6 +533,25 @@ function foo(x) {
         y;
     }
 }
+// We allow an unconstrained object of a generic type `T` to be indexed by a key of type `keyof T`
+// without a check that the object is non-undefined and non-null. This is safe because `keyof T`
+// is `never` (meaning no possible keys) for any `T` that includes `undefined` or `null`.
+function ff1(t, k) {
+    t[k];
+}
+function ff2(t, k) {
+    t[k];
+}
+function ff3(t, k) {
+    t[k]; // Error
+}
+function ff4(t, k) {
+    t[k];
+}
+ff1(null, 'foo'); // Error
+ff2(null, 'foo'); // Error
+ff3(null, 'foo');
+ff4(null, 'foo'); // Error
 
 
 //// [unknownControlFlow.d.ts]
@@ -541,3 +592,12 @@ declare type QQ<T> = NonNullable<NonNullable<NonNullable<T>>>;
 declare function f41<T>(a: T): void;
 declare function deepEquals<T>(a: T, b: T): boolean;
 declare function foo<T>(x: T | null): void;
+declare function ff1<T>(t: T, k: keyof T): void;
+declare function ff2<T>(t: T & {}, k: keyof T): void;
+declare function ff3<T>(t: T, k: keyof (T & {})): void;
+declare function ff4<T>(t: T & {}, k: keyof (T & {})): void;
+declare type Foo = {
+    [key: string]: unknown;
+};
+declare type NullableFoo = Foo | undefined;
+declare type Bar<T extends NullableFoo> = NonNullable<T>[string];

tests/baselines/reference/unknownControlFlow.symbols

@@ -636,3 +636,88 @@ function foo<T>(x: T | null) {
     }
 }
 
+// We allow an unconstrained object of a generic type `T` to be indexed by a key of type `keyof T`
+// without a check that the object is non-undefined and non-null. This is safe because `keyof T`
+// is `never` (meaning no possible keys) for any `T` that includes `undefined` or `null`.
+
+function ff1<T>(t: T, k: keyof T) {
+>ff1 : Symbol(ff1, Decl(unknownControlFlow.ts, 267, 1))
+>T : Symbol(T, Decl(unknownControlFlow.ts, 273, 13))
+>t : Symbol(t, Decl(unknownControlFlow.ts, 273, 16))
+>T : Symbol(T, Decl(unknownControlFlow.ts, 273, 13))
+>k : Symbol(k, Decl(unknownControlFlow.ts, 273, 21))
+>T : Symbol(T, Decl(unknownControlFlow.ts, 273, 13))
+
+    t[k];
+>t : Symbol(t, Decl(unknownControlFlow.ts, 273, 16))
+>k : Symbol(k, Decl(unknownControlFlow.ts, 273, 21))
+}
+
+function ff2<T>(t: T & {}, k: keyof T) {
+>ff2 : Symbol(ff2, Decl(unknownControlFlow.ts, 275, 1))
+>T : Symbol(T, Decl(unknownControlFlow.ts, 277, 13))
+>t : Symbol(t, Decl(unknownControlFlow.ts, 277, 16))
+>T : Symbol(T, Decl(unknownControlFlow.ts, 277, 13))
+>k : Symbol(k, Decl(unknownControlFlow.ts, 277, 26))
+>T : Symbol(T, Decl(unknownControlFlow.ts, 277, 13))
+
+    t[k];
+>t : Symbol(t, Decl(unknownControlFlow.ts, 277, 16))
+>k : Symbol(k, Decl(unknownControlFlow.ts, 277, 26))
+}
+
+function ff3<T>(t: T, k: keyof (T & {})) {
+>ff3 : Symbol(ff3, Decl(unknownControlFlow.ts, 279, 1))
+>T : Symbol(T, Decl(unknownControlFlow.ts, 281, 13))
+>t : Symbol(t, Decl(unknownControlFlow.ts, 281, 16))
+>T : Symbol(T, Decl(unknownControlFlow.ts, 281, 13))
+>k : Symbol(k, Decl(unknownControlFlow.ts, 281, 21))
+>T : Symbol(T, Decl(unknownControlFlow.ts, 281, 13))
+
+    t[k];  // Error
+>t : Symbol(t, Decl(unknownControlFlow.ts, 281, 16))
+>k : Symbol(k, Decl(unknownControlFlow.ts, 281, 21))
+}
+
+function ff4<T>(t: T & {}, k: keyof (T & {})) {
+>ff4 : Symbol(ff4, Decl(unknownControlFlow.ts, 283, 1))
+>T : Symbol(T, Decl(unknownControlFlow.ts, 285, 13))
+>t : Symbol(t, Decl(unknownControlFlow.ts, 285, 16))
+>T : Symbol(T, Decl(unknownControlFlow.ts, 285, 13))
+>k : Symbol(k, Decl(unknownControlFlow.ts, 285, 26))
+>T : Symbol(T, Decl(unknownControlFlow.ts, 285, 13))
+
+    t[k];
+>t : Symbol(t, Decl(unknownControlFlow.ts, 285, 16))
+>k : Symbol(k, Decl(unknownControlFlow.ts, 285, 26))
+}
+
+ff1(null, 'foo');  // Error
+>ff1 : Symbol(ff1, Decl(unknownControlFlow.ts, 267, 1))
+
+ff2(null, 'foo');  // Error
+>ff2 : Symbol(ff2, Decl(unknownControlFlow.ts, 275, 1))
+
+ff3(null, 'foo');
+>ff3 : Symbol(ff3, Decl(unknownControlFlow.ts, 279, 1))
+
+ff4(null, 'foo');  // Error
+>ff4 : Symbol(ff4, Decl(unknownControlFlow.ts, 283, 1))
+
+// Repro from #49681
+
+type Foo = { [key: string]: unknown };
+>Foo : Symbol(Foo, Decl(unknownControlFlow.ts, 292, 17))
+>key : Symbol(key, Decl(unknownControlFlow.ts, 296, 14))
+
+type NullableFoo = Foo | undefined;
+>NullableFoo : Symbol(NullableFoo, Decl(unknownControlFlow.ts, 296, 38))
+>Foo : Symbol(Foo, Decl(unknownControlFlow.ts, 292, 17))
+
+type Bar<T extends NullableFoo> = NonNullable<T>[string];
+>Bar : Symbol(Bar, Decl(unknownControlFlow.ts, 297, 35))
+>T : Symbol(T, Decl(unknownControlFlow.ts, 299, 9))
+>NullableFoo : Symbol(NullableFoo, Decl(unknownControlFlow.ts, 296, 38))
+>NonNullable : Symbol(NonNullable, Decl(lib.es5.d.ts, --, --))
+>T : Symbol(T, Decl(unknownControlFlow.ts, 299, 9))
+

tests/baselines/reference/unknownControlFlow.types

@@ -718,3 +718,87 @@ function foo<T>(x: T | null) {
     }
 }
 
+// We allow an unconstrained object of a generic type `T` to be indexed by a key of type `keyof T`
+// without a check that the object is non-undefined and non-null. This is safe because `keyof T`
+// is `never` (meaning no possible keys) for any `T` that includes `undefined` or `null`.
+
+function ff1<T>(t: T, k: keyof T) {
+>ff1 : <T>(t: T, k: keyof T) => void
+>t : T
+>k : keyof T
+
+    t[k];
+>t[k] : T[keyof T]
+>t : T
+>k : keyof T
+}
+
+function ff2<T>(t: T & {}, k: keyof T) {
+>ff2 : <T>(t: T & {}, k: keyof T) => void
+>t : T & {}
+>k : keyof T
+
+    t[k];
+>t[k] : (T & {})[keyof T]
+>t : T & {}
+>k : keyof T
+}
+
+function ff3<T>(t: T, k: keyof (T & {})) {
+>ff3 : <T>(t: T, k: keyof (T & {})) => void
+>t : T
+>k : keyof (T & {})
+
+    t[k];  // Error
+>t[k] : any
+>t : T
+>k : keyof (T & {})
+}
+
+function ff4<T>(t: T & {}, k: keyof (T & {})) {
+>ff4 : <T>(t: T & {}, k: keyof (T & {})) => void
+>t : T & {}
+>k : keyof (T & {})
+
+    t[k];
+>t[k] : (T & {})[keyof (T & {})]
+>t : T & {}
+>k : keyof (T & {})
+}
+
+ff1(null, 'foo');  // Error
+>ff1(null, 'foo') : void
+>ff1 : <T>(t: T, k: keyof T) => void
+>null : null
+>'foo' : "foo"
+
+ff2(null, 'foo');  // Error
+>ff2(null, 'foo') : void
+>ff2 : <T>(t: T & {}, k: keyof T) => void
+>null : null
+>'foo' : "foo"
+
+ff3(null, 'foo');
+>ff3(null, 'foo') : void
+>ff3 : <T>(t: T, k: keyof (T & {})) => void
+>null : null
+>'foo' : "foo"
+
+ff4(null, 'foo');  // Error
+>ff4(null, 'foo') : void
+>ff4 : <T>(t: T & {}, k: keyof (T & {})) => void
+>null : null
+>'foo' : "foo"
+
+// Repro from #49681
+
+type Foo = { [key: string]: unknown };
+>Foo : { [key: string]: unknown; }
+>key : string
+
+type NullableFoo = Foo | undefined;
+>NullableFoo : Foo | undefined
+
+type Bar<T extends NullableFoo> = NonNullable<T>[string];
+>Bar : Bar<T>
+

tests/cases/conformance/types/mapped/mappedTypes4.ts

@@ -65,7 +65,7 @@ var x1: DeepReadonlyFoo;
 
 type Z = { a: number };
 type Clone<T> = {
-  [P in keyof (T & {})]: T[P];
+  [P in keyof (T & {})]: (T & {})[P];
 };
 type M = Clone<Z>; // M should be { a: number }