From 4fa96056eaf038fa0796c2acb6631f3344603a1c Mon Sep 17 00:00:00 2001
From: Kevin Donnelly <kpdonn@users.noreply.github.com>
Date: Tue, 27 Mar 2018 09:45:25 -0500
Subject: [PATCH]  Make T[never] = never instead of erroring or being any
 (#22787)

* Add tests showing existing behavior for indexing types with never.

* Make T[never] = never instead of erroring or being any.

And update the baselines for the tests for this change.

* Add test case for indexing an expression with never showing existing behavior.

* Make indexing an object with never expression result in never.

And update baseline to reflect new behavior.
---
 src/compiler/checker.ts                       |   3 +
 .../reference/indexingTypesWithNever.js       | 121 ++++++
 .../reference/indexingTypesWithNever.symbols  | 389 +++++++++++++++++
 .../reference/indexingTypesWithNever.types    | 404 ++++++++++++++++++
 .../cases/compiler/indexingTypesWithNever.ts  | 111 +++++
 5 files changed, 1028 insertions(+)
 create mode 100644 tests/baselines/reference/indexingTypesWithNever.js
 create mode 100644 tests/baselines/reference/indexingTypesWithNever.symbols
 create mode 100644 tests/baselines/reference/indexingTypesWithNever.types
 create mode 100644 tests/cases/compiler/indexingTypesWithNever.ts

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 5bf1f022869ee..cc17e30e0e098 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -8151,6 +8151,9 @@ namespace ts {
                     }
                     return indexInfo.type;
                 }
+                if (indexType.flags & TypeFlags.Never) {
+                    return neverType;
+                }
                 if (accessExpression && !isConstEnumObjectType(objectType)) {
                     if (noImplicitAny && !compilerOptions.suppressImplicitAnyIndexErrors) {
                         if (getIndexTypeOfType(objectType, IndexKind.Number)) {
diff --git a/tests/baselines/reference/indexingTypesWithNever.js b/tests/baselines/reference/indexingTypesWithNever.js
new file mode 100644
index 0000000000000..4e174fc4c9d91
--- /dev/null
+++ b/tests/baselines/reference/indexingTypesWithNever.js
@@ -0,0 +1,121 @@
+//// [indexingTypesWithNever.ts]
+type TestObj = {
+  a: string;
+  b: number;
+};
+
+// Should be never but without an error
+type Result1 = TestObj[never];
+
+type EmptyObj = {};
+
+// Should be never but without an error
+type Result2 = EmptyObj[keyof EmptyObj];
+
+declare function genericFn1<T>(obj: T): T[never];
+
+// Should be never
+const result3 = genericFn1({ c: "ctest", d: "dtest" });
+
+declare function genericFn2<T extends { [ind: string]: string }>(
+  obj: T
+): T[never];
+
+// Should be never
+const result4 = genericFn2({ e: "etest", f: "ftest" });
+
+declare function genericFn3<
+  T extends { [K in keyof T]: T[K] },
+  U extends keyof T,
+  V extends keyof T
+>(obj: T, u: U, v: V): T[U & V];
+
+// Should be never
+const result5 = genericFn3({ g: "gtest", h: "htest" }, "g", "h"); // 'g' & 'h' will reduce to never
+
+
+declare const obj: {a: string, b: number}
+declare const key: never
+
+const result6 = obj[key]
+
+// Expanded examples from https://github.com/Microsoft/TypeScript/issues/21988
+type RequiredPropNames<T> = {
+  [P in keyof T]-?: undefined extends T[P] ? never : P
+}[keyof T];
+
+type OptionalPropNames<T> = {
+  [P in keyof T]-?: undefined extends T[P] ? P : never
+}[keyof T];
+
+type RequiredProps<T> = { [P in RequiredPropNames<T>]: T[P] };
+type OptionalProps<T> = { [P in OptionalPropNames<T>]?: T[P] };
+
+type Match<Exp, Act> = [Exp] extends [Act]
+  ? ([Act] extends [Exp] ? "Match" : "Did not match 2")
+  : "Did not match 1";
+
+type ExpectType<Exp, Act> = Match<Exp, Act> extends "Match"
+  ? ({} extends Exp ? Match<Required<Exp>, Required<Act>> : "Match")
+  : "Did not match";
+
+type P3 = { a: string; b: number; c?: boolean };
+type P2 = { a: string; c?: boolean };
+type P1 = { c?: boolean };
+type P0 = {};
+
+type P3Names = RequiredPropNames<P3>; // expect 'a' | 'b'
+type P2Names = RequiredPropNames<P2>; // expect 'a'
+type P1Names = RequiredPropNames<P1>; // expect never
+type P0Names = RequiredPropNames<P0>; // expect never
+
+declare const p3NameTest: ExpectType<"a" | "b", P3Names>;
+declare const p2NameTest: ExpectType<"a", P2Names>;
+declare const p1NameTest: ExpectType<never, P1Names>;
+declare const p0NameTest: ExpectType<never, P0Names>;
+
+type P3Props = RequiredProps<P3>; // expect { a: string; b: number }
+type P2Props = RequiredProps<P2>; // expect { a: string; }
+type P1Props = RequiredProps<P1>; // expect {}
+type P0Props = RequiredProps<P0>; // expect {}
+
+declare const p3Test: ExpectType<{ a: string; b: number }, P3Props>;
+declare const p2Test: ExpectType<{ a: string }, P2Props>;
+declare const p1Test: ExpectType<{}, P1Props>;
+declare const p0Test: ExpectType<{}, P0Props>;
+
+type O3 = { a?: string; b?: number; c: boolean };
+type O2 = { a?: string; c: boolean };
+type O1 = { c: boolean };
+type O0 = {};
+
+type O3Names = OptionalPropNames<O3>; // expect 'a' | 'b'
+type O2Names = OptionalPropNames<O2>; // expect 'a'
+type O1Names = OptionalPropNames<O1>; // expect never
+type O0Names = OptionalPropNames<O0>; // expect never
+
+declare const o3NameTest: ExpectType<"a" | "b", O3Names>;
+declare const o2NameTest: ExpectType<"a", O2Names>;
+declare const o1NameTest: ExpectType<never, O1Names>;
+declare const o0NameTest: ExpectType<never, O0Names>;
+
+type O3Props = OptionalProps<O3>; // expect { a?: string | undefined; b?: number | undefined }
+type O2Props = OptionalProps<O2>; // expect { a?: string | undefined; }
+type O1Props = OptionalProps<O1>; // expect {}
+type O0Props = OptionalProps<O0>; // expect {}
+
+declare const o3Test: ExpectType<{ a?: string; b?: number }, O3Props>;
+declare const o2Test: ExpectType<{ a?: string }, O2Props>;
+declare const o1Test: ExpectType<{}, O1Props>;
+declare const o0Test: ExpectType<{}, O0Props>;
+
+
+//// [indexingTypesWithNever.js]
+"use strict";
+// Should be never
+var result3 = genericFn1({ c: "ctest", d: "dtest" });
+// Should be never
+var result4 = genericFn2({ e: "etest", f: "ftest" });
+// Should be never
+var result5 = genericFn3({ g: "gtest", h: "htest" }, "g", "h"); // 'g' & 'h' will reduce to never
+var result6 = obj[key];
diff --git a/tests/baselines/reference/indexingTypesWithNever.symbols b/tests/baselines/reference/indexingTypesWithNever.symbols
new file mode 100644
index 0000000000000..2b163d74a0260
--- /dev/null
+++ b/tests/baselines/reference/indexingTypesWithNever.symbols
@@ -0,0 +1,389 @@
+=== tests/cases/compiler/indexingTypesWithNever.ts ===
+type TestObj = {
+>TestObj : Symbol(TestObj, Decl(indexingTypesWithNever.ts, 0, 0))
+
+  a: string;
+>a : Symbol(a, Decl(indexingTypesWithNever.ts, 0, 16))
+
+  b: number;
+>b : Symbol(b, Decl(indexingTypesWithNever.ts, 1, 12))
+
+};
+
+// Should be never but without an error
+type Result1 = TestObj[never];
+>Result1 : Symbol(Result1, Decl(indexingTypesWithNever.ts, 3, 2))
+>TestObj : Symbol(TestObj, Decl(indexingTypesWithNever.ts, 0, 0))
+
+type EmptyObj = {};
+>EmptyObj : Symbol(EmptyObj, Decl(indexingTypesWithNever.ts, 6, 30))
+
+// Should be never but without an error
+type Result2 = EmptyObj[keyof EmptyObj];
+>Result2 : Symbol(Result2, Decl(indexingTypesWithNever.ts, 8, 19))
+>EmptyObj : Symbol(EmptyObj, Decl(indexingTypesWithNever.ts, 6, 30))
+>EmptyObj : Symbol(EmptyObj, Decl(indexingTypesWithNever.ts, 6, 30))
+
+declare function genericFn1<T>(obj: T): T[never];
+>genericFn1 : Symbol(genericFn1, Decl(indexingTypesWithNever.ts, 11, 40))
+>T : Symbol(T, Decl(indexingTypesWithNever.ts, 13, 28))
+>obj : Symbol(obj, Decl(indexingTypesWithNever.ts, 13, 31))
+>T : Symbol(T, Decl(indexingTypesWithNever.ts, 13, 28))
+>T : Symbol(T, Decl(indexingTypesWithNever.ts, 13, 28))
+
+// Should be never
+const result3 = genericFn1({ c: "ctest", d: "dtest" });
+>result3 : Symbol(result3, Decl(indexingTypesWithNever.ts, 16, 5))
+>genericFn1 : Symbol(genericFn1, Decl(indexingTypesWithNever.ts, 11, 40))
+>c : Symbol(c, Decl(indexingTypesWithNever.ts, 16, 28))
+>d : Symbol(d, Decl(indexingTypesWithNever.ts, 16, 40))
+
+declare function genericFn2<T extends { [ind: string]: string }>(
+>genericFn2 : Symbol(genericFn2, Decl(indexingTypesWithNever.ts, 16, 55))
+>T : Symbol(T, Decl(indexingTypesWithNever.ts, 18, 28))
+>ind : Symbol(ind, Decl(indexingTypesWithNever.ts, 18, 41))
+
+  obj: T
+>obj : Symbol(obj, Decl(indexingTypesWithNever.ts, 18, 65))
+>T : Symbol(T, Decl(indexingTypesWithNever.ts, 18, 28))
+
+): T[never];
+>T : Symbol(T, Decl(indexingTypesWithNever.ts, 18, 28))
+
+// Should be never
+const result4 = genericFn2({ e: "etest", f: "ftest" });
+>result4 : Symbol(result4, Decl(indexingTypesWithNever.ts, 23, 5))
+>genericFn2 : Symbol(genericFn2, Decl(indexingTypesWithNever.ts, 16, 55))
+>e : Symbol(e, Decl(indexingTypesWithNever.ts, 23, 28))
+>f : Symbol(f, Decl(indexingTypesWithNever.ts, 23, 40))
+
+declare function genericFn3<
+>genericFn3 : Symbol(genericFn3, Decl(indexingTypesWithNever.ts, 23, 55))
+
+  T extends { [K in keyof T]: T[K] },
+>T : Symbol(T, Decl(indexingTypesWithNever.ts, 25, 28))
+>K : Symbol(K, Decl(indexingTypesWithNever.ts, 26, 15))
+>T : Symbol(T, Decl(indexingTypesWithNever.ts, 25, 28))
+>T : Symbol(T, Decl(indexingTypesWithNever.ts, 25, 28))
+>K : Symbol(K, Decl(indexingTypesWithNever.ts, 26, 15))
+
+  U extends keyof T,
+>U : Symbol(U, Decl(indexingTypesWithNever.ts, 26, 37))
+>T : Symbol(T, Decl(indexingTypesWithNever.ts, 25, 28))
+
+  V extends keyof T
+>V : Symbol(V, Decl(indexingTypesWithNever.ts, 27, 20))
+>T : Symbol(T, Decl(indexingTypesWithNever.ts, 25, 28))
+
+>(obj: T, u: U, v: V): T[U & V];
+>obj : Symbol(obj, Decl(indexingTypesWithNever.ts, 29, 2))
+>T : Symbol(T, Decl(indexingTypesWithNever.ts, 25, 28))
+>u : Symbol(u, Decl(indexingTypesWithNever.ts, 29, 9))
+>U : Symbol(U, Decl(indexingTypesWithNever.ts, 26, 37))
+>v : Symbol(v, Decl(indexingTypesWithNever.ts, 29, 15))
+>V : Symbol(V, Decl(indexingTypesWithNever.ts, 27, 20))
+>T : Symbol(T, Decl(indexingTypesWithNever.ts, 25, 28))
+>U : Symbol(U, Decl(indexingTypesWithNever.ts, 26, 37))
+>V : Symbol(V, Decl(indexingTypesWithNever.ts, 27, 20))
+
+// Should be never
+const result5 = genericFn3({ g: "gtest", h: "htest" }, "g", "h"); // 'g' & 'h' will reduce to never
+>result5 : Symbol(result5, Decl(indexingTypesWithNever.ts, 32, 5))
+>genericFn3 : Symbol(genericFn3, Decl(indexingTypesWithNever.ts, 23, 55))
+>g : Symbol(g, Decl(indexingTypesWithNever.ts, 32, 28))
+>h : Symbol(h, Decl(indexingTypesWithNever.ts, 32, 40))
+
+
+declare const obj: {a: string, b: number}
+>obj : Symbol(obj, Decl(indexingTypesWithNever.ts, 35, 13))
+>a : Symbol(a, Decl(indexingTypesWithNever.ts, 35, 20))
+>b : Symbol(b, Decl(indexingTypesWithNever.ts, 35, 30))
+
+declare const key: never
+>key : Symbol(key, Decl(indexingTypesWithNever.ts, 36, 13))
+
+const result6 = obj[key]
+>result6 : Symbol(result6, Decl(indexingTypesWithNever.ts, 38, 5))
+>obj : Symbol(obj, Decl(indexingTypesWithNever.ts, 35, 13))
+>key : Symbol(key, Decl(indexingTypesWithNever.ts, 36, 13))
+
+// Expanded examples from https://github.com/Microsoft/TypeScript/issues/21988
+type RequiredPropNames<T> = {
+>RequiredPropNames : Symbol(RequiredPropNames, Decl(indexingTypesWithNever.ts, 38, 24))
+>T : Symbol(T, Decl(indexingTypesWithNever.ts, 41, 23))
+
+  [P in keyof T]-?: undefined extends T[P] ? never : P
+>P : Symbol(P, Decl(indexingTypesWithNever.ts, 42, 3))
+>T : Symbol(T, Decl(indexingTypesWithNever.ts, 41, 23))
+>T : Symbol(T, Decl(indexingTypesWithNever.ts, 41, 23))
+>P : Symbol(P, Decl(indexingTypesWithNever.ts, 42, 3))
+>P : Symbol(P, Decl(indexingTypesWithNever.ts, 42, 3))
+
+}[keyof T];
+>T : Symbol(T, Decl(indexingTypesWithNever.ts, 41, 23))
+
+type OptionalPropNames<T> = {
+>OptionalPropNames : Symbol(OptionalPropNames, Decl(indexingTypesWithNever.ts, 43, 11))
+>T : Symbol(T, Decl(indexingTypesWithNever.ts, 45, 23))
+
+  [P in keyof T]-?: undefined extends T[P] ? P : never
+>P : Symbol(P, Decl(indexingTypesWithNever.ts, 46, 3))
+>T : Symbol(T, Decl(indexingTypesWithNever.ts, 45, 23))
+>T : Symbol(T, Decl(indexingTypesWithNever.ts, 45, 23))
+>P : Symbol(P, Decl(indexingTypesWithNever.ts, 46, 3))
+>P : Symbol(P, Decl(indexingTypesWithNever.ts, 46, 3))
+
+}[keyof T];
+>T : Symbol(T, Decl(indexingTypesWithNever.ts, 45, 23))
+
+type RequiredProps<T> = { [P in RequiredPropNames<T>]: T[P] };
+>RequiredProps : Symbol(RequiredProps, Decl(indexingTypesWithNever.ts, 47, 11))
+>T : Symbol(T, Decl(indexingTypesWithNever.ts, 49, 19))
+>P : Symbol(P, Decl(indexingTypesWithNever.ts, 49, 27))
+>RequiredPropNames : Symbol(RequiredPropNames, Decl(indexingTypesWithNever.ts, 38, 24))
+>T : Symbol(T, Decl(indexingTypesWithNever.ts, 49, 19))
+>T : Symbol(T, Decl(indexingTypesWithNever.ts, 49, 19))
+>P : Symbol(P, Decl(indexingTypesWithNever.ts, 49, 27))
+
+type OptionalProps<T> = { [P in OptionalPropNames<T>]?: T[P] };
+>OptionalProps : Symbol(OptionalProps, Decl(indexingTypesWithNever.ts, 49, 62))
+>T : Symbol(T, Decl(indexingTypesWithNever.ts, 50, 19))
+>P : Symbol(P, Decl(indexingTypesWithNever.ts, 50, 27))
+>OptionalPropNames : Symbol(OptionalPropNames, Decl(indexingTypesWithNever.ts, 43, 11))
+>T : Symbol(T, Decl(indexingTypesWithNever.ts, 50, 19))
+>T : Symbol(T, Decl(indexingTypesWithNever.ts, 50, 19))
+>P : Symbol(P, Decl(indexingTypesWithNever.ts, 50, 27))
+
+type Match<Exp, Act> = [Exp] extends [Act]
+>Match : Symbol(Match, Decl(indexingTypesWithNever.ts, 50, 63))
+>Exp : Symbol(Exp, Decl(indexingTypesWithNever.ts, 52, 11))
+>Act : Symbol(Act, Decl(indexingTypesWithNever.ts, 52, 15))
+>Exp : Symbol(Exp, Decl(indexingTypesWithNever.ts, 52, 11))
+>Act : Symbol(Act, Decl(indexingTypesWithNever.ts, 52, 15))
+
+  ? ([Act] extends [Exp] ? "Match" : "Did not match 2")
+>Act : Symbol(Act, Decl(indexingTypesWithNever.ts, 52, 15))
+>Exp : Symbol(Exp, Decl(indexingTypesWithNever.ts, 52, 11))
+
+  : "Did not match 1";
+
+type ExpectType<Exp, Act> = Match<Exp, Act> extends "Match"
+>ExpectType : Symbol(ExpectType, Decl(indexingTypesWithNever.ts, 54, 22))
+>Exp : Symbol(Exp, Decl(indexingTypesWithNever.ts, 56, 16))
+>Act : Symbol(Act, Decl(indexingTypesWithNever.ts, 56, 20))
+>Match : Symbol(Match, Decl(indexingTypesWithNever.ts, 50, 63))
+>Exp : Symbol(Exp, Decl(indexingTypesWithNever.ts, 56, 16))
+>Act : Symbol(Act, Decl(indexingTypesWithNever.ts, 56, 20))
+
+  ? ({} extends Exp ? Match<Required<Exp>, Required<Act>> : "Match")
+>Exp : Symbol(Exp, Decl(indexingTypesWithNever.ts, 56, 16))
+>Match : Symbol(Match, Decl(indexingTypesWithNever.ts, 50, 63))
+>Required : Symbol(Required, Decl(lib.d.ts, --, --))
+>Exp : Symbol(Exp, Decl(indexingTypesWithNever.ts, 56, 16))
+>Required : Symbol(Required, Decl(lib.d.ts, --, --))
+>Act : Symbol(Act, Decl(indexingTypesWithNever.ts, 56, 20))
+
+  : "Did not match";
+
+type P3 = { a: string; b: number; c?: boolean };
+>P3 : Symbol(P3, Decl(indexingTypesWithNever.ts, 58, 20))
+>a : Symbol(a, Decl(indexingTypesWithNever.ts, 60, 11))
+>b : Symbol(b, Decl(indexingTypesWithNever.ts, 60, 22))
+>c : Symbol(c, Decl(indexingTypesWithNever.ts, 60, 33))
+
+type P2 = { a: string; c?: boolean };
+>P2 : Symbol(P2, Decl(indexingTypesWithNever.ts, 60, 48))
+>a : Symbol(a, Decl(indexingTypesWithNever.ts, 61, 11))
+>c : Symbol(c, Decl(indexingTypesWithNever.ts, 61, 22))
+
+type P1 = { c?: boolean };
+>P1 : Symbol(P1, Decl(indexingTypesWithNever.ts, 61, 37))
+>c : Symbol(c, Decl(indexingTypesWithNever.ts, 62, 11))
+
+type P0 = {};
+>P0 : Symbol(P0, Decl(indexingTypesWithNever.ts, 62, 26))
+
+type P3Names = RequiredPropNames<P3>; // expect 'a' | 'b'
+>P3Names : Symbol(P3Names, Decl(indexingTypesWithNever.ts, 63, 13))
+>RequiredPropNames : Symbol(RequiredPropNames, Decl(indexingTypesWithNever.ts, 38, 24))
+>P3 : Symbol(P3, Decl(indexingTypesWithNever.ts, 58, 20))
+
+type P2Names = RequiredPropNames<P2>; // expect 'a'
+>P2Names : Symbol(P2Names, Decl(indexingTypesWithNever.ts, 65, 37))
+>RequiredPropNames : Symbol(RequiredPropNames, Decl(indexingTypesWithNever.ts, 38, 24))
+>P2 : Symbol(P2, Decl(indexingTypesWithNever.ts, 60, 48))
+
+type P1Names = RequiredPropNames<P1>; // expect never
+>P1Names : Symbol(P1Names, Decl(indexingTypesWithNever.ts, 66, 37))
+>RequiredPropNames : Symbol(RequiredPropNames, Decl(indexingTypesWithNever.ts, 38, 24))
+>P1 : Symbol(P1, Decl(indexingTypesWithNever.ts, 61, 37))
+
+type P0Names = RequiredPropNames<P0>; // expect never
+>P0Names : Symbol(P0Names, Decl(indexingTypesWithNever.ts, 67, 37))
+>RequiredPropNames : Symbol(RequiredPropNames, Decl(indexingTypesWithNever.ts, 38, 24))
+>P0 : Symbol(P0, Decl(indexingTypesWithNever.ts, 62, 26))
+
+declare const p3NameTest: ExpectType<"a" | "b", P3Names>;
+>p3NameTest : Symbol(p3NameTest, Decl(indexingTypesWithNever.ts, 70, 13))
+>ExpectType : Symbol(ExpectType, Decl(indexingTypesWithNever.ts, 54, 22))
+>P3Names : Symbol(P3Names, Decl(indexingTypesWithNever.ts, 63, 13))
+
+declare const p2NameTest: ExpectType<"a", P2Names>;
+>p2NameTest : Symbol(p2NameTest, Decl(indexingTypesWithNever.ts, 71, 13))
+>ExpectType : Symbol(ExpectType, Decl(indexingTypesWithNever.ts, 54, 22))
+>P2Names : Symbol(P2Names, Decl(indexingTypesWithNever.ts, 65, 37))
+
+declare const p1NameTest: ExpectType<never, P1Names>;
+>p1NameTest : Symbol(p1NameTest, Decl(indexingTypesWithNever.ts, 72, 13))
+>ExpectType : Symbol(ExpectType, Decl(indexingTypesWithNever.ts, 54, 22))
+>P1Names : Symbol(P1Names, Decl(indexingTypesWithNever.ts, 66, 37))
+
+declare const p0NameTest: ExpectType<never, P0Names>;
+>p0NameTest : Symbol(p0NameTest, Decl(indexingTypesWithNever.ts, 73, 13))
+>ExpectType : Symbol(ExpectType, Decl(indexingTypesWithNever.ts, 54, 22))
+>P0Names : Symbol(P0Names, Decl(indexingTypesWithNever.ts, 67, 37))
+
+type P3Props = RequiredProps<P3>; // expect { a: string; b: number }
+>P3Props : Symbol(P3Props, Decl(indexingTypesWithNever.ts, 73, 53))
+>RequiredProps : Symbol(RequiredProps, Decl(indexingTypesWithNever.ts, 47, 11))
+>P3 : Symbol(P3, Decl(indexingTypesWithNever.ts, 58, 20))
+
+type P2Props = RequiredProps<P2>; // expect { a: string; }
+>P2Props : Symbol(P2Props, Decl(indexingTypesWithNever.ts, 75, 33))
+>RequiredProps : Symbol(RequiredProps, Decl(indexingTypesWithNever.ts, 47, 11))
+>P2 : Symbol(P2, Decl(indexingTypesWithNever.ts, 60, 48))
+
+type P1Props = RequiredProps<P1>; // expect {}
+>P1Props : Symbol(P1Props, Decl(indexingTypesWithNever.ts, 76, 33))
+>RequiredProps : Symbol(RequiredProps, Decl(indexingTypesWithNever.ts, 47, 11))
+>P1 : Symbol(P1, Decl(indexingTypesWithNever.ts, 61, 37))
+
+type P0Props = RequiredProps<P0>; // expect {}
+>P0Props : Symbol(P0Props, Decl(indexingTypesWithNever.ts, 77, 33))
+>RequiredProps : Symbol(RequiredProps, Decl(indexingTypesWithNever.ts, 47, 11))
+>P0 : Symbol(P0, Decl(indexingTypesWithNever.ts, 62, 26))
+
+declare const p3Test: ExpectType<{ a: string; b: number }, P3Props>;
+>p3Test : Symbol(p3Test, Decl(indexingTypesWithNever.ts, 80, 13))
+>ExpectType : Symbol(ExpectType, Decl(indexingTypesWithNever.ts, 54, 22))
+>a : Symbol(a, Decl(indexingTypesWithNever.ts, 80, 34))
+>b : Symbol(b, Decl(indexingTypesWithNever.ts, 80, 45))
+>P3Props : Symbol(P3Props, Decl(indexingTypesWithNever.ts, 73, 53))
+
+declare const p2Test: ExpectType<{ a: string }, P2Props>;
+>p2Test : Symbol(p2Test, Decl(indexingTypesWithNever.ts, 81, 13))
+>ExpectType : Symbol(ExpectType, Decl(indexingTypesWithNever.ts, 54, 22))
+>a : Symbol(a, Decl(indexingTypesWithNever.ts, 81, 34))
+>P2Props : Symbol(P2Props, Decl(indexingTypesWithNever.ts, 75, 33))
+
+declare const p1Test: ExpectType<{}, P1Props>;
+>p1Test : Symbol(p1Test, Decl(indexingTypesWithNever.ts, 82, 13))
+>ExpectType : Symbol(ExpectType, Decl(indexingTypesWithNever.ts, 54, 22))
+>P1Props : Symbol(P1Props, Decl(indexingTypesWithNever.ts, 76, 33))
+
+declare const p0Test: ExpectType<{}, P0Props>;
+>p0Test : Symbol(p0Test, Decl(indexingTypesWithNever.ts, 83, 13))
+>ExpectType : Symbol(ExpectType, Decl(indexingTypesWithNever.ts, 54, 22))
+>P0Props : Symbol(P0Props, Decl(indexingTypesWithNever.ts, 77, 33))
+
+type O3 = { a?: string; b?: number; c: boolean };
+>O3 : Symbol(O3, Decl(indexingTypesWithNever.ts, 83, 46))
+>a : Symbol(a, Decl(indexingTypesWithNever.ts, 85, 11))
+>b : Symbol(b, Decl(indexingTypesWithNever.ts, 85, 23))
+>c : Symbol(c, Decl(indexingTypesWithNever.ts, 85, 35))
+
+type O2 = { a?: string; c: boolean };
+>O2 : Symbol(O2, Decl(indexingTypesWithNever.ts, 85, 49))
+>a : Symbol(a, Decl(indexingTypesWithNever.ts, 86, 11))
+>c : Symbol(c, Decl(indexingTypesWithNever.ts, 86, 23))
+
+type O1 = { c: boolean };
+>O1 : Symbol(O1, Decl(indexingTypesWithNever.ts, 86, 37))
+>c : Symbol(c, Decl(indexingTypesWithNever.ts, 87, 11))
+
+type O0 = {};
+>O0 : Symbol(O0, Decl(indexingTypesWithNever.ts, 87, 25))
+
+type O3Names = OptionalPropNames<O3>; // expect 'a' | 'b'
+>O3Names : Symbol(O3Names, Decl(indexingTypesWithNever.ts, 88, 13))
+>OptionalPropNames : Symbol(OptionalPropNames, Decl(indexingTypesWithNever.ts, 43, 11))
+>O3 : Symbol(O3, Decl(indexingTypesWithNever.ts, 83, 46))
+
+type O2Names = OptionalPropNames<O2>; // expect 'a'
+>O2Names : Symbol(O2Names, Decl(indexingTypesWithNever.ts, 90, 37))
+>OptionalPropNames : Symbol(OptionalPropNames, Decl(indexingTypesWithNever.ts, 43, 11))
+>O2 : Symbol(O2, Decl(indexingTypesWithNever.ts, 85, 49))
+
+type O1Names = OptionalPropNames<O1>; // expect never
+>O1Names : Symbol(O1Names, Decl(indexingTypesWithNever.ts, 91, 37))
+>OptionalPropNames : Symbol(OptionalPropNames, Decl(indexingTypesWithNever.ts, 43, 11))
+>O1 : Symbol(O1, Decl(indexingTypesWithNever.ts, 86, 37))
+
+type O0Names = OptionalPropNames<O0>; // expect never
+>O0Names : Symbol(O0Names, Decl(indexingTypesWithNever.ts, 92, 37))
+>OptionalPropNames : Symbol(OptionalPropNames, Decl(indexingTypesWithNever.ts, 43, 11))
+>O0 : Symbol(O0, Decl(indexingTypesWithNever.ts, 87, 25))
+
+declare const o3NameTest: ExpectType<"a" | "b", O3Names>;
+>o3NameTest : Symbol(o3NameTest, Decl(indexingTypesWithNever.ts, 95, 13))
+>ExpectType : Symbol(ExpectType, Decl(indexingTypesWithNever.ts, 54, 22))
+>O3Names : Symbol(O3Names, Decl(indexingTypesWithNever.ts, 88, 13))
+
+declare const o2NameTest: ExpectType<"a", O2Names>;
+>o2NameTest : Symbol(o2NameTest, Decl(indexingTypesWithNever.ts, 96, 13))
+>ExpectType : Symbol(ExpectType, Decl(indexingTypesWithNever.ts, 54, 22))
+>O2Names : Symbol(O2Names, Decl(indexingTypesWithNever.ts, 90, 37))
+
+declare const o1NameTest: ExpectType<never, O1Names>;
+>o1NameTest : Symbol(o1NameTest, Decl(indexingTypesWithNever.ts, 97, 13))
+>ExpectType : Symbol(ExpectType, Decl(indexingTypesWithNever.ts, 54, 22))
+>O1Names : Symbol(O1Names, Decl(indexingTypesWithNever.ts, 91, 37))
+
+declare const o0NameTest: ExpectType<never, O0Names>;
+>o0NameTest : Symbol(o0NameTest, Decl(indexingTypesWithNever.ts, 98, 13))
+>ExpectType : Symbol(ExpectType, Decl(indexingTypesWithNever.ts, 54, 22))
+>O0Names : Symbol(O0Names, Decl(indexingTypesWithNever.ts, 92, 37))
+
+type O3Props = OptionalProps<O3>; // expect { a?: string | undefined; b?: number | undefined }
+>O3Props : Symbol(O3Props, Decl(indexingTypesWithNever.ts, 98, 53))
+>OptionalProps : Symbol(OptionalProps, Decl(indexingTypesWithNever.ts, 49, 62))
+>O3 : Symbol(O3, Decl(indexingTypesWithNever.ts, 83, 46))
+
+type O2Props = OptionalProps<O2>; // expect { a?: string | undefined; }
+>O2Props : Symbol(O2Props, Decl(indexingTypesWithNever.ts, 100, 33))
+>OptionalProps : Symbol(OptionalProps, Decl(indexingTypesWithNever.ts, 49, 62))
+>O2 : Symbol(O2, Decl(indexingTypesWithNever.ts, 85, 49))
+
+type O1Props = OptionalProps<O1>; // expect {}
+>O1Props : Symbol(O1Props, Decl(indexingTypesWithNever.ts, 101, 33))
+>OptionalProps : Symbol(OptionalProps, Decl(indexingTypesWithNever.ts, 49, 62))
+>O1 : Symbol(O1, Decl(indexingTypesWithNever.ts, 86, 37))
+
+type O0Props = OptionalProps<O0>; // expect {}
+>O0Props : Symbol(O0Props, Decl(indexingTypesWithNever.ts, 102, 33))
+>OptionalProps : Symbol(OptionalProps, Decl(indexingTypesWithNever.ts, 49, 62))
+>O0 : Symbol(O0, Decl(indexingTypesWithNever.ts, 87, 25))
+
+declare const o3Test: ExpectType<{ a?: string; b?: number }, O3Props>;
+>o3Test : Symbol(o3Test, Decl(indexingTypesWithNever.ts, 105, 13))
+>ExpectType : Symbol(ExpectType, Decl(indexingTypesWithNever.ts, 54, 22))
+>a : Symbol(a, Decl(indexingTypesWithNever.ts, 105, 34))
+>b : Symbol(b, Decl(indexingTypesWithNever.ts, 105, 46))
+>O3Props : Symbol(O3Props, Decl(indexingTypesWithNever.ts, 98, 53))
+
+declare const o2Test: ExpectType<{ a?: string }, O2Props>;
+>o2Test : Symbol(o2Test, Decl(indexingTypesWithNever.ts, 106, 13))
+>ExpectType : Symbol(ExpectType, Decl(indexingTypesWithNever.ts, 54, 22))
+>a : Symbol(a, Decl(indexingTypesWithNever.ts, 106, 34))
+>O2Props : Symbol(O2Props, Decl(indexingTypesWithNever.ts, 100, 33))
+
+declare const o1Test: ExpectType<{}, O1Props>;
+>o1Test : Symbol(o1Test, Decl(indexingTypesWithNever.ts, 107, 13))
+>ExpectType : Symbol(ExpectType, Decl(indexingTypesWithNever.ts, 54, 22))
+>O1Props : Symbol(O1Props, Decl(indexingTypesWithNever.ts, 101, 33))
+
+declare const o0Test: ExpectType<{}, O0Props>;
+>o0Test : Symbol(o0Test, Decl(indexingTypesWithNever.ts, 108, 13))
+>ExpectType : Symbol(ExpectType, Decl(indexingTypesWithNever.ts, 54, 22))
+>O0Props : Symbol(O0Props, Decl(indexingTypesWithNever.ts, 102, 33))
+
diff --git a/tests/baselines/reference/indexingTypesWithNever.types b/tests/baselines/reference/indexingTypesWithNever.types
new file mode 100644
index 0000000000000..fb6b3904f3a5a
--- /dev/null
+++ b/tests/baselines/reference/indexingTypesWithNever.types
@@ -0,0 +1,404 @@
+=== tests/cases/compiler/indexingTypesWithNever.ts ===
+type TestObj = {
+>TestObj : TestObj
+
+  a: string;
+>a : string
+
+  b: number;
+>b : number
+
+};
+
+// Should be never but without an error
+type Result1 = TestObj[never];
+>Result1 : never
+>TestObj : TestObj
+
+type EmptyObj = {};
+>EmptyObj : EmptyObj
+
+// Should be never but without an error
+type Result2 = EmptyObj[keyof EmptyObj];
+>Result2 : never
+>EmptyObj : EmptyObj
+>EmptyObj : EmptyObj
+
+declare function genericFn1<T>(obj: T): T[never];
+>genericFn1 : <T>(obj: T) => T[never]
+>T : T
+>obj : T
+>T : T
+>T : T
+
+// Should be never
+const result3 = genericFn1({ c: "ctest", d: "dtest" });
+>result3 : never
+>genericFn1({ c: "ctest", d: "dtest" }) : never
+>genericFn1 : <T>(obj: T) => T[never]
+>{ c: "ctest", d: "dtest" } : { c: string; d: string; }
+>c : string
+>"ctest" : "ctest"
+>d : string
+>"dtest" : "dtest"
+
+declare function genericFn2<T extends { [ind: string]: string }>(
+>genericFn2 : <T extends { [ind: string]: string; }>(obj: T) => T[never]
+>T : T
+>ind : string
+
+  obj: T
+>obj : T
+>T : T
+
+): T[never];
+>T : T
+
+// Should be never
+const result4 = genericFn2({ e: "etest", f: "ftest" });
+>result4 : never
+>genericFn2({ e: "etest", f: "ftest" }) : never
+>genericFn2 : <T extends { [ind: string]: string; }>(obj: T) => T[never]
+>{ e: "etest", f: "ftest" } : { e: string; f: string; }
+>e : string
+>"etest" : "etest"
+>f : string
+>"ftest" : "ftest"
+
+declare function genericFn3<
+>genericFn3 : <T extends { [K in keyof T]: T[K]; }, U extends keyof T, V extends keyof T>(obj: T, u: U, v: V) => T[U & V]
+
+  T extends { [K in keyof T]: T[K] },
+>T : T
+>K : K
+>T : T
+>T : T
+>K : K
+
+  U extends keyof T,
+>U : U
+>T : T
+
+  V extends keyof T
+>V : V
+>T : T
+
+>(obj: T, u: U, v: V): T[U & V];
+>obj : T
+>T : T
+>u : U
+>U : U
+>v : V
+>V : V
+>T : T
+>U : U
+>V : V
+
+// Should be never
+const result5 = genericFn3({ g: "gtest", h: "htest" }, "g", "h"); // 'g' & 'h' will reduce to never
+>result5 : any
+>genericFn3({ g: "gtest", h: "htest" }, "g", "h") : any
+>genericFn3 : <T extends { [K in keyof T]: T[K]; }, U extends keyof T, V extends keyof T>(obj: T, u: U, v: V) => T[U & V]
+>{ g: "gtest", h: "htest" } : { g: string; h: string; }
+>g : string
+>"gtest" : "gtest"
+>h : string
+>"htest" : "htest"
+>"g" : "g"
+>"h" : "h"
+
+
+declare const obj: {a: string, b: number}
+>obj : { a: string; b: number; }
+>a : string
+>b : number
+
+declare const key: never
+>key : never
+
+const result6 = obj[key]
+>result6 : never
+>obj[key] : never
+>obj : { a: string; b: number; }
+>key : never
+
+// Expanded examples from https://github.com/Microsoft/TypeScript/issues/21988
+type RequiredPropNames<T> = {
+>RequiredPropNames : { [P in keyof T]-?: undefined extends T[P] ? never : P; }[keyof T]
+>T : T
+
+  [P in keyof T]-?: undefined extends T[P] ? never : P
+>P : P
+>T : T
+>T : T
+>P : P
+>P : P
+
+}[keyof T];
+>T : T
+
+type OptionalPropNames<T> = {
+>OptionalPropNames : { [P in keyof T]-?: undefined extends T[P] ? P : never; }[keyof T]
+>T : T
+
+  [P in keyof T]-?: undefined extends T[P] ? P : never
+>P : P
+>T : T
+>T : T
+>P : P
+>P : P
+
+}[keyof T];
+>T : T
+
+type RequiredProps<T> = { [P in RequiredPropNames<T>]: T[P] };
+>RequiredProps : RequiredProps<T>
+>T : T
+>P : P
+>RequiredPropNames : { [P in keyof T]-?: undefined extends T[P] ? never : P; }[keyof T]
+>T : T
+>T : T
+>P : P
+
+type OptionalProps<T> = { [P in OptionalPropNames<T>]?: T[P] };
+>OptionalProps : OptionalProps<T>
+>T : T
+>P : P
+>OptionalPropNames : { [P in keyof T]-?: undefined extends T[P] ? P : never; }[keyof T]
+>T : T
+>T : T
+>P : P
+
+type Match<Exp, Act> = [Exp] extends [Act]
+>Match : Match<Exp, Act>
+>Exp : Exp
+>Act : Act
+>Exp : Exp
+>Act : Act
+
+  ? ([Act] extends [Exp] ? "Match" : "Did not match 2")
+>Act : Act
+>Exp : Exp
+
+  : "Did not match 1";
+
+type ExpectType<Exp, Act> = Match<Exp, Act> extends "Match"
+>ExpectType : ExpectType<Exp, Act>
+>Exp : Exp
+>Act : Act
+>Match : Match<Exp, Act>
+>Exp : Exp
+>Act : Act
+
+  ? ({} extends Exp ? Match<Required<Exp>, Required<Act>> : "Match")
+>Exp : Exp
+>Match : Match<Exp, Act>
+>Required : Required<T>
+>Exp : Exp
+>Required : Required<T>
+>Act : Act
+
+  : "Did not match";
+
+type P3 = { a: string; b: number; c?: boolean };
+>P3 : P3
+>a : string
+>b : number
+>c : boolean | undefined
+
+type P2 = { a: string; c?: boolean };
+>P2 : P2
+>a : string
+>c : boolean | undefined
+
+type P1 = { c?: boolean };
+>P1 : P1
+>c : boolean | undefined
+
+type P0 = {};
+>P0 : P0
+
+type P3Names = RequiredPropNames<P3>; // expect 'a' | 'b'
+>P3Names : "a" | "b"
+>RequiredPropNames : { [P in keyof T]-?: undefined extends T[P] ? never : P; }[keyof T]
+>P3 : P3
+
+type P2Names = RequiredPropNames<P2>; // expect 'a'
+>P2Names : "a"
+>RequiredPropNames : { [P in keyof T]-?: undefined extends T[P] ? never : P; }[keyof T]
+>P2 : P2
+
+type P1Names = RequiredPropNames<P1>; // expect never
+>P1Names : never
+>RequiredPropNames : { [P in keyof T]-?: undefined extends T[P] ? never : P; }[keyof T]
+>P1 : P1
+
+type P0Names = RequiredPropNames<P0>; // expect never
+>P0Names : never
+>RequiredPropNames : { [P in keyof T]-?: undefined extends T[P] ? never : P; }[keyof T]
+>P0 : P0
+
+declare const p3NameTest: ExpectType<"a" | "b", P3Names>;
+>p3NameTest : "Match"
+>ExpectType : ExpectType<Exp, Act>
+>P3Names : "a" | "b"
+
+declare const p2NameTest: ExpectType<"a", P2Names>;
+>p2NameTest : "Match"
+>ExpectType : ExpectType<Exp, Act>
+>P2Names : "a"
+
+declare const p1NameTest: ExpectType<never, P1Names>;
+>p1NameTest : "Match"
+>ExpectType : ExpectType<Exp, Act>
+>P1Names : never
+
+declare const p0NameTest: ExpectType<never, P0Names>;
+>p0NameTest : "Match"
+>ExpectType : ExpectType<Exp, Act>
+>P0Names : never
+
+type P3Props = RequiredProps<P3>; // expect { a: string; b: number }
+>P3Props : RequiredProps<P3>
+>RequiredProps : RequiredProps<T>
+>P3 : P3
+
+type P2Props = RequiredProps<P2>; // expect { a: string; }
+>P2Props : RequiredProps<P2>
+>RequiredProps : RequiredProps<T>
+>P2 : P2
+
+type P1Props = RequiredProps<P1>; // expect {}
+>P1Props : RequiredProps<P1>
+>RequiredProps : RequiredProps<T>
+>P1 : P1
+
+type P0Props = RequiredProps<P0>; // expect {}
+>P0Props : RequiredProps<P0>
+>RequiredProps : RequiredProps<T>
+>P0 : P0
+
+declare const p3Test: ExpectType<{ a: string; b: number }, P3Props>;
+>p3Test : "Match"
+>ExpectType : ExpectType<Exp, Act>
+>a : string
+>b : number
+>P3Props : RequiredProps<P3>
+
+declare const p2Test: ExpectType<{ a: string }, P2Props>;
+>p2Test : "Match"
+>ExpectType : ExpectType<Exp, Act>
+>a : string
+>P2Props : RequiredProps<P2>
+
+declare const p1Test: ExpectType<{}, P1Props>;
+>p1Test : "Match"
+>ExpectType : ExpectType<Exp, Act>
+>P1Props : RequiredProps<P1>
+
+declare const p0Test: ExpectType<{}, P0Props>;
+>p0Test : "Match"
+>ExpectType : ExpectType<Exp, Act>
+>P0Props : RequiredProps<P0>
+
+type O3 = { a?: string; b?: number; c: boolean };
+>O3 : O3
+>a : string | undefined
+>b : number | undefined
+>c : boolean
+
+type O2 = { a?: string; c: boolean };
+>O2 : O2
+>a : string | undefined
+>c : boolean
+
+type O1 = { c: boolean };
+>O1 : O1
+>c : boolean
+
+type O0 = {};
+>O0 : O0
+
+type O3Names = OptionalPropNames<O3>; // expect 'a' | 'b'
+>O3Names : "a" | "b"
+>OptionalPropNames : { [P in keyof T]-?: undefined extends T[P] ? P : never; }[keyof T]
+>O3 : O3
+
+type O2Names = OptionalPropNames<O2>; // expect 'a'
+>O2Names : "a"
+>OptionalPropNames : { [P in keyof T]-?: undefined extends T[P] ? P : never; }[keyof T]
+>O2 : O2
+
+type O1Names = OptionalPropNames<O1>; // expect never
+>O1Names : never
+>OptionalPropNames : { [P in keyof T]-?: undefined extends T[P] ? P : never; }[keyof T]
+>O1 : O1
+
+type O0Names = OptionalPropNames<O0>; // expect never
+>O0Names : never
+>OptionalPropNames : { [P in keyof T]-?: undefined extends T[P] ? P : never; }[keyof T]
+>O0 : O0
+
+declare const o3NameTest: ExpectType<"a" | "b", O3Names>;
+>o3NameTest : "Match"
+>ExpectType : ExpectType<Exp, Act>
+>O3Names : "a" | "b"
+
+declare const o2NameTest: ExpectType<"a", O2Names>;
+>o2NameTest : "Match"
+>ExpectType : ExpectType<Exp, Act>
+>O2Names : "a"
+
+declare const o1NameTest: ExpectType<never, O1Names>;
+>o1NameTest : "Match"
+>ExpectType : ExpectType<Exp, Act>
+>O1Names : never
+
+declare const o0NameTest: ExpectType<never, O0Names>;
+>o0NameTest : "Match"
+>ExpectType : ExpectType<Exp, Act>
+>O0Names : never
+
+type O3Props = OptionalProps<O3>; // expect { a?: string | undefined; b?: number | undefined }
+>O3Props : OptionalProps<O3>
+>OptionalProps : OptionalProps<T>
+>O3 : O3
+
+type O2Props = OptionalProps<O2>; // expect { a?: string | undefined; }
+>O2Props : OptionalProps<O2>
+>OptionalProps : OptionalProps<T>
+>O2 : O2
+
+type O1Props = OptionalProps<O1>; // expect {}
+>O1Props : OptionalProps<O1>
+>OptionalProps : OptionalProps<T>
+>O1 : O1
+
+type O0Props = OptionalProps<O0>; // expect {}
+>O0Props : OptionalProps<O0>
+>OptionalProps : OptionalProps<T>
+>O0 : O0
+
+declare const o3Test: ExpectType<{ a?: string; b?: number }, O3Props>;
+>o3Test : "Match"
+>ExpectType : ExpectType<Exp, Act>
+>a : string | undefined
+>b : number | undefined
+>O3Props : OptionalProps<O3>
+
+declare const o2Test: ExpectType<{ a?: string }, O2Props>;
+>o2Test : "Match"
+>ExpectType : ExpectType<Exp, Act>
+>a : string | undefined
+>O2Props : OptionalProps<O2>
+
+declare const o1Test: ExpectType<{}, O1Props>;
+>o1Test : "Match"
+>ExpectType : ExpectType<Exp, Act>
+>O1Props : OptionalProps<O1>
+
+declare const o0Test: ExpectType<{}, O0Props>;
+>o0Test : "Match"
+>ExpectType : ExpectType<Exp, Act>
+>O0Props : OptionalProps<O0>
+
diff --git a/tests/cases/compiler/indexingTypesWithNever.ts b/tests/cases/compiler/indexingTypesWithNever.ts
new file mode 100644
index 0000000000000..9044ded2ff4c3
--- /dev/null
+++ b/tests/cases/compiler/indexingTypesWithNever.ts
@@ -0,0 +1,111 @@
+// @strict: true
+
+type TestObj = {
+  a: string;
+  b: number;
+};
+
+// Should be never but without an error
+type Result1 = TestObj[never];
+
+type EmptyObj = {};
+
+// Should be never but without an error
+type Result2 = EmptyObj[keyof EmptyObj];
+
+declare function genericFn1<T>(obj: T): T[never];
+
+// Should be never
+const result3 = genericFn1({ c: "ctest", d: "dtest" });
+
+declare function genericFn2<T extends { [ind: string]: string }>(
+  obj: T
+): T[never];
+
+// Should be never
+const result4 = genericFn2({ e: "etest", f: "ftest" });
+
+declare function genericFn3<
+  T extends { [K in keyof T]: T[K] },
+  U extends keyof T,
+  V extends keyof T
+>(obj: T, u: U, v: V): T[U & V];
+
+// Should be never
+const result5 = genericFn3({ g: "gtest", h: "htest" }, "g", "h"); // 'g' & 'h' will reduce to never
+
+
+declare const obj: {a: string, b: number}
+declare const key: never
+
+const result6 = obj[key]
+
+// Expanded examples from https://github.com/Microsoft/TypeScript/issues/21988
+type RequiredPropNames<T> = {
+  [P in keyof T]-?: undefined extends T[P] ? never : P
+}[keyof T];
+
+type OptionalPropNames<T> = {
+  [P in keyof T]-?: undefined extends T[P] ? P : never
+}[keyof T];
+
+type RequiredProps<T> = { [P in RequiredPropNames<T>]: T[P] };
+type OptionalProps<T> = { [P in OptionalPropNames<T>]?: T[P] };
+
+type Match<Exp, Act> = [Exp] extends [Act]
+  ? ([Act] extends [Exp] ? "Match" : "Did not match 2")
+  : "Did not match 1";
+
+type ExpectType<Exp, Act> = Match<Exp, Act> extends "Match"
+  ? ({} extends Exp ? Match<Required<Exp>, Required<Act>> : "Match")
+  : "Did not match";
+
+type P3 = { a: string; b: number; c?: boolean };
+type P2 = { a: string; c?: boolean };
+type P1 = { c?: boolean };
+type P0 = {};
+
+type P3Names = RequiredPropNames<P3>; // expect 'a' | 'b'
+type P2Names = RequiredPropNames<P2>; // expect 'a'
+type P1Names = RequiredPropNames<P1>; // expect never
+type P0Names = RequiredPropNames<P0>; // expect never
+
+declare const p3NameTest: ExpectType<"a" | "b", P3Names>;
+declare const p2NameTest: ExpectType<"a", P2Names>;
+declare const p1NameTest: ExpectType<never, P1Names>;
+declare const p0NameTest: ExpectType<never, P0Names>;
+
+type P3Props = RequiredProps<P3>; // expect { a: string; b: number }
+type P2Props = RequiredProps<P2>; // expect { a: string; }
+type P1Props = RequiredProps<P1>; // expect {}
+type P0Props = RequiredProps<P0>; // expect {}
+
+declare const p3Test: ExpectType<{ a: string; b: number }, P3Props>;
+declare const p2Test: ExpectType<{ a: string }, P2Props>;
+declare const p1Test: ExpectType<{}, P1Props>;
+declare const p0Test: ExpectType<{}, P0Props>;
+
+type O3 = { a?: string; b?: number; c: boolean };
+type O2 = { a?: string; c: boolean };
+type O1 = { c: boolean };
+type O0 = {};
+
+type O3Names = OptionalPropNames<O3>; // expect 'a' | 'b'
+type O2Names = OptionalPropNames<O2>; // expect 'a'
+type O1Names = OptionalPropNames<O1>; // expect never
+type O0Names = OptionalPropNames<O0>; // expect never
+
+declare const o3NameTest: ExpectType<"a" | "b", O3Names>;
+declare const o2NameTest: ExpectType<"a", O2Names>;
+declare const o1NameTest: ExpectType<never, O1Names>;
+declare const o0NameTest: ExpectType<never, O0Names>;
+
+type O3Props = OptionalProps<O3>; // expect { a?: string | undefined; b?: number | undefined }
+type O2Props = OptionalProps<O2>; // expect { a?: string | undefined; }
+type O1Props = OptionalProps<O1>; // expect {}
+type O0Props = OptionalProps<O0>; // expect {}
+
+declare const o3Test: ExpectType<{ a?: string; b?: number }, O3Props>;
+declare const o2Test: ExpectType<{ a?: string }, O2Props>;
+declare const o1Test: ExpectType<{}, O1Props>;
+declare const o0Test: ExpectType<{}, O0Props>;