Improve intersection reduction and CFA for truthy, equality, and typeof checks (microsoft#49119)

* Improve reduction of intersection types

* Accept new baselines

* Improve CFA for truthy, equality, and typeof checks

* Accept new baselines

* Remove special case for Function type

* Don't reduce intersections of form {...} & object

* Accept new baselines

* Anything is assignable to unknown-like union

* Accept new baselines

* Tweak subtype check

* Recombine unknown type from unknown-like union in more cases

* Display union origin only if it is shorter than union itself

* Accept new baselines

* Add tests

* Only attach origin type when it is shorter than union itself

* Specially preserve string & {}, number & {}, bigint & {}

* Accept new baselines

* Add additional tests

* Fix getNormalizedType and getNarrowableTypeForReference for intersections

* Switch NonNullable<T> to use T & {}

* Accept new baselines

* Use NonNullable<T> in place of anonymous T & {}

* Accept new baselines

* Add fourslash test

* More fourslash tests

* Fix getFalsyFlags handling of intersections

* Accept new baselines

* Add constraint to compareProperties type parameter

* Unconstrained type parameter not assignable to {} with strictNullChecks

* Accept new baselines

Author: ahejlsberg

src/compiler/checker.ts

@@ -2035,7 +2035,7 @@ namespace ts {
         return comparer(a, b);
     }
 
-    export function compareProperties<T, K extends keyof T>(a: T | undefined, b: T | undefined, key: K, comparer: Comparer<T[K]>): Comparison {
+    export function compareProperties<T extends object, K extends keyof T>(a: T | undefined, b: T | undefined, key: K, comparer: Comparer<T[K]>): Comparison {
         return a === b ? Comparison.EqualTo :
             a === undefined ? Comparison.LessThan :
             b === undefined ? Comparison.GreaterThan :

src/compiler/core.ts

@@ -1526,6 +1526,10 @@
         "category": "Error",
         "code": 2207
     },
+    "This type parameter probably needs an `extends object` constraint.": {
+        "category": "Error",
+        "code": 2208
+    },
 
     "The project root is ambiguous, but is required to resolve export map entry '{0}' in file '{1}'. Supply the `rootDir` compiler option to disambiguate.": {
         "category": "Error",

src/compiler/diagnosticMessages.json

@@ -5397,6 +5397,11 @@ namespace ts {
         // Flags that require TypeFlags.Union
         /* @internal */
         ContainsIntersections = 1 << 24, // Union contains intersections
+        /* @internal */
+        IsUnknownLikeUnionComputed = 1 << 25, // IsUnknownLikeUnion flag has been computed
+        /* @internal */
+        IsUnknownLikeUnion = 1 << 26, // Union of null, undefined, and empty object type
+        /* @internal */
 
         // Flags that require TypeFlags.Intersection
         /* @internal */

src/compiler/types.ts

@@ -1564,7 +1564,7 @@ type Omit<T, K extends keyof any> = Pick<T, Exclude<keyof T, K>>;
 /**
  * Exclude null and undefined from T
  */
-type NonNullable<T> = T extends null | undefined ? never : T;
+type NonNullable<T> = T & {};
 
 /**
  * Obtain the parameters of a function type in a tuple

src/lib/es5.d.ts

@@ -150,7 +150,7 @@ function f01(x: unknown) {
 >undefined : undefined
 >typeof x === "string" : boolean
 >typeof x : "string" | "number" | "bigint" | "boolean" | "symbol" | "undefined" | "object" | "function"
->x : unknown
+>x : {} | null
 >"string" : "string"
 
         x;  // string | undefined

tests/baselines/reference/assertionTypePredicates1.types

@@ -1,12 +1,23 @@
 tests/cases/conformance/types/conditional/conditionalTypes1.ts(12,5): error TS2322: Type 'T' is not assignable to type 'NonNullable<T>'.
+  Type 'T' is not assignable to type '{}'.
 tests/cases/conformance/types/conditional/conditionalTypes1.ts(17,5): error TS2322: Type 'T' is not assignable to type 'NonNullable<T>'.
   Type 'string | undefined' is not assignable to type 'NonNullable<T>'.
     Type 'undefined' is not assignable to type 'NonNullable<T>'.
+      Type 'undefined' is not assignable to type 'T'.
+        'undefined' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint 'string | undefined'.
+          Type 'T' is not assignable to type '{}'.
+            Type 'string | undefined' is not assignable to type '{}'.
+              Type 'undefined' is not assignable to type '{}'.
 tests/cases/conformance/types/conditional/conditionalTypes1.ts(24,5): error TS2322: Type 'T[keyof T] | undefined' is not assignable to type 'NonNullable<Partial<T>[keyof T]>'.
-  Type '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]'.
 tests/cases/conformance/types/conditional/conditionalTypes1.ts(29,5): error TS2322: Type 'T["x"]' is not assignable to type 'NonNullable<T["x"]>'.
   Type 'string | undefined' is not assignable to type 'NonNullable<T["x"]>'.
     Type 'undefined' is not assignable to type 'NonNullable<T["x"]>'.
+      Type 'undefined' is not assignable to type '{}'.
+        Type 'T["x"]' is not assignable to type '{}'.
+          Type 'string | undefined' is not assignable to type '{}'.
+            Type 'undefined' is not assignable to type '{}'.
 tests/cases/conformance/types/conditional/conditionalTypes1.ts(103,5): error TS2322: Type 'FunctionProperties<T>' is not assignable to type 'T'.
   'T' could be instantiated with an arbitrary type which could be unrelated to 'FunctionProperties<T>'.
 tests/cases/conformance/types/conditional/conditionalTypes1.ts(104,5): error TS2322: Type 'NonFunctionProperties<T>' is not assignable to type 'T'.
@@ -72,6 +83,8 @@ tests/cases/conformance/types/conditional/conditionalTypes1.ts(288,43): error TS
         y = x;  // Error
         ~
 !!! error TS2322: Type 'T' is not assignable to type 'NonNullable<T>'.
+!!! error TS2322:   Type 'T' is not assignable to type '{}'.
+!!! related TS2208 tests/cases/conformance/types/conditional/conditionalTypes1.ts:10:13: This type parameter probably needs an `extends object` constraint.
     }
 
     function f2<T extends string | undefined>(x: T, y: NonNullable<T>) {
@@ -81,6 +94,11 @@ tests/cases/conformance/types/conditional/conditionalTypes1.ts(288,43): error TS
 !!! error TS2322: Type 'T' is not assignable to type 'NonNullable<T>'.
 !!! error TS2322:   Type 'string | undefined' is not assignable to type 'NonNullable<T>'.
 !!! error TS2322:     Type 'undefined' is not assignable to type 'NonNullable<T>'.
+!!! error TS2322:       Type 'undefined' is not assignable to type 'T'.
+!!! error TS2322:         'undefined' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint 'string | undefined'.
+!!! error TS2322:           Type 'T' is not assignable to type '{}'.
+!!! error TS2322:             Type 'string | undefined' is not assignable to type '{}'.
+!!! error TS2322:               Type 'undefined' is not assignable to type '{}'.
         let s1: string = x;  // Error
         let s2: string = y;
     }
@@ -90,7 +108,8 @@ tests/cases/conformance/types/conditional/conditionalTypes1.ts(288,43): error TS
         y = x;  // Error
         ~
 !!! error TS2322: Type 'T[keyof T] | undefined' is not assignable to type 'NonNullable<Partial<T>[keyof T]>'.
-!!! error TS2322:   Type '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]'.
     }
 
     function f4<T extends { x: string | undefined }>(x: T["x"], y: NonNullable<T["x"]>) {
@@ -100,6 +119,10 @@ tests/cases/conformance/types/conditional/conditionalTypes1.ts(288,43): error TS
 !!! error TS2322: Type 'T["x"]' is not assignable to type 'NonNullable<T["x"]>'.
 !!! error TS2322:   Type 'string | undefined' is not assignable to type 'NonNullable<T["x"]>'.
 !!! error TS2322:     Type 'undefined' is not assignable to type 'NonNullable<T["x"]>'.
+!!! error TS2322:       Type 'undefined' is not assignable to type '{}'.
+!!! error TS2322:         Type 'T["x"]' is not assignable to type '{}'.
+!!! error TS2322:           Type 'string | undefined' is not assignable to type '{}'.
+!!! error TS2322:             Type 'undefined' is not assignable to type '{}'.
         let s1: string = x;  // Error
         let s2: string = y;
     }

tests/baselines/reference/conditionalTypes1.errors.txt

@@ -55,7 +55,7 @@ function f2<T extends string | undefined>(x: T, y: NonNullable<T>) {
 
     let s2: string = y;
 >s2 : string
->y : NonNullable<T>
+>y : string
 }
 
 function f3<T>(x: Partial<T>[keyof T], y: NonNullable<Partial<T>[keyof T]>) {
@@ -96,7 +96,7 @@ function f4<T extends { x: string | undefined }>(x: T["x"], y: NonNullable<T["x"
 
     let s2: string = y;
 >s2 : string
->y : NonNullable<T["x"]>
+>y : string
 }
 
 type Options = { k: "a", a: number } | { k: "b", b: string } | { k: "c", c: boolean };

tests/baselines/reference/conditionalTypes1.types

@@ -32,7 +32,7 @@ tests/cases/conformance/types/conditional/conditionalTypes2.ts(74,12): error TS2
   Property 'bat' is missing in type 'Foo & Bar' but required in type '{ foo: string; bat: string; }'.
 tests/cases/conformance/types/conditional/conditionalTypes2.ts(75,12): error TS2345: Argument of type 'Extract2<T, Foo, Bar>' is not assignable to parameter of type '{ foo: string; bat: string; }'.
   Type 'T extends Bar ? T : never' is not assignable to type '{ foo: string; bat: string; }'.
-    Property 'bat' is missing in type 'Bar & Foo' but required in type '{ foo: string; bat: string; }'.
+    Type 'Bar & Foo & T' is not assignable to type '{ foo: string; bat: string; }'.
 
 
 ==== tests/cases/conformance/types/conditional/conditionalTypes2.ts (7 errors) ====
@@ -155,8 +155,7 @@ tests/cases/conformance/types/conditional/conditionalTypes2.ts(75,12): error TS2
                ~
 !!! error TS2345: Argument of type 'Extract2<T, Foo, Bar>' is not assignable to parameter of type '{ foo: string; bat: string; }'.
 !!! error TS2345:   Type 'T extends Bar ? T : never' is not assignable to type '{ foo: string; bat: string; }'.
-!!! error TS2345:     Property 'bat' is missing in type 'Bar & Foo' but required in type '{ foo: string; bat: string; }'.
-!!! related TS2728 tests/cases/conformance/types/conditional/conditionalTypes2.ts:62:43: 'bat' is declared here.
+!!! error TS2345:     Type 'Bar & Foo & T' is not assignable to type '{ foo: string; bat: string; }'.
     }
 
     // Repros from #22860

tests/baselines/reference/conditionalTypes2.errors.txt

@@ -10,7 +10,7 @@ function f1<T extends string | undefined>(x: T, y: { a: T }, z: [T]): string {
 >x : T
 
         x;
->x : T
+>x : NonNullable<T>
 
         x.length;
 >x.length : number
@@ -67,7 +67,7 @@ function f2<T>(x: Extract<T, string | undefined> | null): string {
 >x : Extract<T, string | undefined> | null
 
         x;
->x : Extract<T, string | undefined>
+>x : NonNullable<Extract<T, string | undefined>>
 
         x.length;
 >x.length : number
@@ -404,11 +404,11 @@ function fx1<T, K extends keyof T>(obj: T, key: K) {
 >key : K
 
     const x2 = obj && obj[key];
->x2 : T[K]
->obj && obj[key] : T[K]
->obj : T
->obj[key] : T[K]
+>x2 : NonNullable<T>[K]
+>obj && obj[key] : NonNullable<T>[K]
 >obj : T
+>obj[key] : NonNullable<T>[K]
+>obj : NonNullable<T>
 >key : K
 }
 
@@ -438,8 +438,8 @@ function fx3<T extends Record<keyof T, string> | undefined, K extends keyof T>(o
 >key : K
 
     const x1 = obj[key];  // Error
->x1 : NonNullable<Record<keyof T, string>>[K]
->obj[key] : NonNullable<Record<keyof T, string>>[K]
+>x1 : Record<keyof T, string>[K]
+>obj[key] : Record<keyof T, string>[K]
 >obj : Record<keyof T, string> | undefined
 >key : K
 
@@ -469,14 +469,14 @@ class TableBaseEnum<
 >null : null
 
         iSpec[null! as keyof InternalSpec];  // Error, object possibly undefined
->iSpec[null! as keyof InternalSpec] : NonNullable<Record<keyof PublicSpec, any>>[keyof InternalSpec]
+>iSpec[null! as keyof InternalSpec] : Record<keyof PublicSpec, any>[keyof InternalSpec]
 >iSpec : Record<keyof PublicSpec, any> | undefined
 >null! as keyof InternalSpec : keyof InternalSpec
 >null! : never
 >null : null
 
         iSpec[null! as keyof PublicSpec];    // Error, object possibly undefined
->iSpec[null! as keyof PublicSpec] : NonNullable<Record<keyof PublicSpec, any>>[keyof PublicSpec]
+>iSpec[null! as keyof PublicSpec] : Record<keyof PublicSpec, any>[keyof PublicSpec]
 >iSpec : Record<keyof PublicSpec, any> | undefined
 >null! as keyof PublicSpec : keyof PublicSpec
 >null! : never