Cherry-pick PR #50553 into release-4.8 (#50589)

Component commits:
3652682 {} & null and {} & undefined should be never in non-strictNullChecks mode

dd32cf8 Add tests

03cb73f Address code review feedback

4502553 Accept new baselines

Co-authored-by: Anders Hejlsberg <[email protected]>

src/compiler/checker.ts

@@ -15134,7 +15134,7 @@ namespace ts {
                 return includes & TypeFlags.IncludesWildcard ? wildcardType : anyType;
             }
             if (!strictNullChecks && includes & TypeFlags.Nullable) {
-                return includes & TypeFlags.Undefined ? undefinedType : nullType;
+                return includes & TypeFlags.IncludesEmptyObject ? neverType : includes & TypeFlags.Undefined ? undefinedType : nullType;
             }
             if (includes & TypeFlags.String && includes & (TypeFlags.StringLiteral | TypeFlags.TemplateLiteral | TypeFlags.StringMapping) ||
                 includes & TypeFlags.Number && includes & TypeFlags.NumberLiteral ||

tests/baselines/reference/NonNullableInNonStrictMode.js

@@ -0,0 +1,21 @@
+//// [NonNullableInNonStrictMode.ts]
+// These should all resolve to never
+
+type T0 = NonNullable<null>;
+type T1 = NonNullable<undefined>;
+type T2 = null & {};
+type T3 = undefined & {};
+type T4 = null & undefined;
+type T6 = null & { a: string } & {};
+
+// Repro from #50519
+
+type NonNullableNew<T> = T & {};
+type NonNullableOld<T> = T extends null | undefined ? never : T;
+
+type IsNullWithoutStrictNullChecks = NonNullableNew<null>;
+type IsAlwaysNever = NonNullableOld<null>;
+
+
+//// [NonNullableInNonStrictMode.js]
+// These should all resolve to never

tests/baselines/reference/NonNullableInNonStrictMode.symbols

@@ -0,0 +1,45 @@
+=== tests/cases/compiler/NonNullableInNonStrictMode.ts ===
+// These should all resolve to never
+
+type T0 = NonNullable<null>;
+>T0 : Symbol(T0, Decl(NonNullableInNonStrictMode.ts, 0, 0))
+>NonNullable : Symbol(NonNullable, Decl(lib.es5.d.ts, --, --))
+
+type T1 = NonNullable<undefined>;
+>T1 : Symbol(T1, Decl(NonNullableInNonStrictMode.ts, 2, 28))
+>NonNullable : Symbol(NonNullable, Decl(lib.es5.d.ts, --, --))
+
+type T2 = null & {};
+>T2 : Symbol(T2, Decl(NonNullableInNonStrictMode.ts, 3, 33))
+
+type T3 = undefined & {};
+>T3 : Symbol(T3, Decl(NonNullableInNonStrictMode.ts, 4, 20))
+
+type T4 = null & undefined;
+>T4 : Symbol(T4, Decl(NonNullableInNonStrictMode.ts, 5, 25))
+
+type T6 = null & { a: string } & {};
+>T6 : Symbol(T6, Decl(NonNullableInNonStrictMode.ts, 6, 27))
+>a : Symbol(a, Decl(NonNullableInNonStrictMode.ts, 7, 18))
+
+// Repro from #50519
+
+type NonNullableNew<T> = T & {};
+>NonNullableNew : Symbol(NonNullableNew, Decl(NonNullableInNonStrictMode.ts, 7, 36))
+>T : Symbol(T, Decl(NonNullableInNonStrictMode.ts, 11, 20))
+>T : Symbol(T, Decl(NonNullableInNonStrictMode.ts, 11, 20))
+
+type NonNullableOld<T> = T extends null | undefined ? never : T;
+>NonNullableOld : Symbol(NonNullableOld, Decl(NonNullableInNonStrictMode.ts, 11, 32))
+>T : Symbol(T, Decl(NonNullableInNonStrictMode.ts, 12, 20))
+>T : Symbol(T, Decl(NonNullableInNonStrictMode.ts, 12, 20))
+>T : Symbol(T, Decl(NonNullableInNonStrictMode.ts, 12, 20))
+
+type IsNullWithoutStrictNullChecks = NonNullableNew<null>;
+>IsNullWithoutStrictNullChecks : Symbol(IsNullWithoutStrictNullChecks, Decl(NonNullableInNonStrictMode.ts, 12, 64))
+>NonNullableNew : Symbol(NonNullableNew, Decl(NonNullableInNonStrictMode.ts, 7, 36))
+
+type IsAlwaysNever = NonNullableOld<null>;
+>IsAlwaysNever : Symbol(IsAlwaysNever, Decl(NonNullableInNonStrictMode.ts, 14, 58))
+>NonNullableOld : Symbol(NonNullableOld, Decl(NonNullableInNonStrictMode.ts, 11, 32))
+

tests/baselines/reference/NonNullableInNonStrictMode.types

@@ -0,0 +1,43 @@
+=== tests/cases/compiler/NonNullableInNonStrictMode.ts ===
+// These should all resolve to never
+
+type T0 = NonNullable<null>;
+>T0 : never
+>null : null
+
+type T1 = NonNullable<undefined>;
+>T1 : never
+
+type T2 = null & {};
+>T2 : never
+>null : null
+
+type T3 = undefined & {};
+>T3 : never
+
+type T4 = null & undefined;
+>T4 : never
+>null : null
+
+type T6 = null & { a: string } & {};
+>T6 : never
+>null : null
+>a : string
+
+// Repro from #50519
+
+type NonNullableNew<T> = T & {};
+>NonNullableNew : NonNullableNew<T>
+
+type NonNullableOld<T> = T extends null | undefined ? never : T;
+>NonNullableOld : NonNullableOld<T>
+>null : null
+
+type IsNullWithoutStrictNullChecks = NonNullableNew<null>;
+>IsNullWithoutStrictNullChecks : never
+>null : null
+
+type IsAlwaysNever = NonNullableOld<null>;
+>IsAlwaysNever : never
+>null : null
+

tests/baselines/reference/nonNullableAndObjectIntersections(strict=false).js

@@ -0,0 +1,21 @@
+//// [nonNullableAndObjectIntersections.ts]
+// These should all resolve to never
+
+type T0 = NonNullable<null>;
+type T1 = NonNullable<undefined>;
+type T2 = null & {};
+type T3 = undefined & {};
+type T4 = null & undefined;
+type T6 = null & { a: string } & {};
+
+// Repro from #50519
+
+type NonNullableNew<T> = T & {};
+type NonNullableOld<T> = T extends null | undefined ? never : T;
+
+type TestNew = NonNullableNew<null>;
+type TestOld = NonNullableOld<null>;
+
+
+//// [nonNullableAndObjectIntersections.js]
+// These should all resolve to never

tests/baselines/reference/nonNullableAndObjectIntersections(strict=false).symbols

@@ -0,0 +1,45 @@
+=== tests/cases/compiler/nonNullableAndObjectIntersections.ts ===
+// These should all resolve to never
+
+type T0 = NonNullable<null>;
+>T0 : Symbol(T0, Decl(nonNullableAndObjectIntersections.ts, 0, 0))
+>NonNullable : Symbol(NonNullable, Decl(lib.es5.d.ts, --, --))
+
+type T1 = NonNullable<undefined>;
+>T1 : Symbol(T1, Decl(nonNullableAndObjectIntersections.ts, 2, 28))
+>NonNullable : Symbol(NonNullable, Decl(lib.es5.d.ts, --, --))
+
+type T2 = null & {};
+>T2 : Symbol(T2, Decl(nonNullableAndObjectIntersections.ts, 3, 33))
+
+type T3 = undefined & {};
+>T3 : Symbol(T3, Decl(nonNullableAndObjectIntersections.ts, 4, 20))
+
+type T4 = null & undefined;
+>T4 : Symbol(T4, Decl(nonNullableAndObjectIntersections.ts, 5, 25))
+
+type T6 = null & { a: string } & {};
+>T6 : Symbol(T6, Decl(nonNullableAndObjectIntersections.ts, 6, 27))
+>a : Symbol(a, Decl(nonNullableAndObjectIntersections.ts, 7, 18))
+
+// Repro from #50519
+
+type NonNullableNew<T> = T & {};
+>NonNullableNew : Symbol(NonNullableNew, Decl(nonNullableAndObjectIntersections.ts, 7, 36))
+>T : Symbol(T, Decl(nonNullableAndObjectIntersections.ts, 11, 20))
+>T : Symbol(T, Decl(nonNullableAndObjectIntersections.ts, 11, 20))
+
+type NonNullableOld<T> = T extends null | undefined ? never : T;
+>NonNullableOld : Symbol(NonNullableOld, Decl(nonNullableAndObjectIntersections.ts, 11, 32))
+>T : Symbol(T, Decl(nonNullableAndObjectIntersections.ts, 12, 20))
+>T : Symbol(T, Decl(nonNullableAndObjectIntersections.ts, 12, 20))
+>T : Symbol(T, Decl(nonNullableAndObjectIntersections.ts, 12, 20))
+
+type TestNew = NonNullableNew<null>;
+>TestNew : Symbol(TestNew, Decl(nonNullableAndObjectIntersections.ts, 12, 64))
+>NonNullableNew : Symbol(NonNullableNew, Decl(nonNullableAndObjectIntersections.ts, 7, 36))
+
+type TestOld = NonNullableOld<null>;
+>TestOld : Symbol(TestOld, Decl(nonNullableAndObjectIntersections.ts, 14, 36))
+>NonNullableOld : Symbol(NonNullableOld, Decl(nonNullableAndObjectIntersections.ts, 11, 32))
+

tests/baselines/reference/nonNullableAndObjectIntersections(strict=false).types

@@ -0,0 +1,43 @@
+=== tests/cases/compiler/nonNullableAndObjectIntersections.ts ===
+// These should all resolve to never
+
+type T0 = NonNullable<null>;
+>T0 : never
+>null : null
+
+type T1 = NonNullable<undefined>;
+>T1 : never
+
+type T2 = null & {};
+>T2 : never
+>null : null
+
+type T3 = undefined & {};
+>T3 : never
+
+type T4 = null & undefined;
+>T4 : never
+>null : null
+
+type T6 = null & { a: string } & {};
+>T6 : never
+>null : null
+>a : string
+
+// Repro from #50519
+
+type NonNullableNew<T> = T & {};
+>NonNullableNew : NonNullableNew<T>
+
+type NonNullableOld<T> = T extends null | undefined ? never : T;
+>NonNullableOld : NonNullableOld<T>
+>null : null
+
+type TestNew = NonNullableNew<null>;
+>TestNew : never
+>null : null
+
+type TestOld = NonNullableOld<null>;
+>TestOld : never
+>null : null
+

tests/baselines/reference/nonNullableAndObjectIntersections(strict=true).js

@@ -0,0 +1,22 @@
+//// [nonNullableAndObjectIntersections.ts]
+// These should all resolve to never
+
+type T0 = NonNullable<null>;
+type T1 = NonNullable<undefined>;
+type T2 = null & {};
+type T3 = undefined & {};
+type T4 = null & undefined;
+type T6 = null & { a: string } & {};
+
+// Repro from #50519
+
+type NonNullableNew<T> = T & {};
+type NonNullableOld<T> = T extends null | undefined ? never : T;
+
+type TestNew = NonNullableNew<null>;
+type TestOld = NonNullableOld<null>;
+
+
+//// [nonNullableAndObjectIntersections.js]
+"use strict";
+// These should all resolve to never

tests/baselines/reference/nonNullableAndObjectIntersections(strict=true).symbols

@@ -0,0 +1,45 @@
+=== tests/cases/compiler/nonNullableAndObjectIntersections.ts ===
+// These should all resolve to never
+
+type T0 = NonNullable<null>;
+>T0 : Symbol(T0, Decl(nonNullableAndObjectIntersections.ts, 0, 0))
+>NonNullable : Symbol(NonNullable, Decl(lib.es5.d.ts, --, --))
+
+type T1 = NonNullable<undefined>;
+>T1 : Symbol(T1, Decl(nonNullableAndObjectIntersections.ts, 2, 28))
+>NonNullable : Symbol(NonNullable, Decl(lib.es5.d.ts, --, --))
+
+type T2 = null & {};
+>T2 : Symbol(T2, Decl(nonNullableAndObjectIntersections.ts, 3, 33))
+
+type T3 = undefined & {};
+>T3 : Symbol(T3, Decl(nonNullableAndObjectIntersections.ts, 4, 20))
+
+type T4 = null & undefined;
+>T4 : Symbol(T4, Decl(nonNullableAndObjectIntersections.ts, 5, 25))
+
+type T6 = null & { a: string } & {};
+>T6 : Symbol(T6, Decl(nonNullableAndObjectIntersections.ts, 6, 27))
+>a : Symbol(a, Decl(nonNullableAndObjectIntersections.ts, 7, 18))
+
+// Repro from #50519
+
+type NonNullableNew<T> = T & {};
+>NonNullableNew : Symbol(NonNullableNew, Decl(nonNullableAndObjectIntersections.ts, 7, 36))
+>T : Symbol(T, Decl(nonNullableAndObjectIntersections.ts, 11, 20))
+>T : Symbol(T, Decl(nonNullableAndObjectIntersections.ts, 11, 20))
+
+type NonNullableOld<T> = T extends null | undefined ? never : T;
+>NonNullableOld : Symbol(NonNullableOld, Decl(nonNullableAndObjectIntersections.ts, 11, 32))
+>T : Symbol(T, Decl(nonNullableAndObjectIntersections.ts, 12, 20))
+>T : Symbol(T, Decl(nonNullableAndObjectIntersections.ts, 12, 20))
+>T : Symbol(T, Decl(nonNullableAndObjectIntersections.ts, 12, 20))
+
+type TestNew = NonNullableNew<null>;
+>TestNew : Symbol(TestNew, Decl(nonNullableAndObjectIntersections.ts, 12, 64))
+>NonNullableNew : Symbol(NonNullableNew, Decl(nonNullableAndObjectIntersections.ts, 7, 36))
+
+type TestOld = NonNullableOld<null>;
+>TestOld : Symbol(TestOld, Decl(nonNullableAndObjectIntersections.ts, 14, 36))
+>NonNullableOld : Symbol(NonNullableOld, Decl(nonNullableAndObjectIntersections.ts, 11, 32))
+

tests/baselines/reference/nonNullableAndObjectIntersections(strict=true).types

@@ -0,0 +1,43 @@
+=== tests/cases/compiler/nonNullableAndObjectIntersections.ts ===
+// These should all resolve to never
+
+type T0 = NonNullable<null>;
+>T0 : never
+>null : null
+
+type T1 = NonNullable<undefined>;
+>T1 : never
+
+type T2 = null & {};
+>T2 : never
+>null : null
+
+type T3 = undefined & {};
+>T3 : never
+
+type T4 = null & undefined;
+>T4 : never
+>null : null
+
+type T6 = null & { a: string } & {};
+>T6 : never
+>null : null
+>a : string
+
+// Repro from #50519
+
+type NonNullableNew<T> = T & {};
+>NonNullableNew : NonNullableNew<T>
+
+type NonNullableOld<T> = T extends null | undefined ? never : T;
+>NonNullableOld : NonNullableOld<T>
+>null : null
+
+type TestNew = NonNullableNew<null>;
+>TestNew : never
+>null : null
+
+type TestOld = NonNullableOld<null>;
+>TestOld : never
+>null : null
+

tests/cases/compiler/nonNullableAndObjectIntersections.ts

@@ -0,0 +1,18 @@
+// @strict: true, false
+
+// These should all resolve to never
+
+type T0 = NonNullable<null>;
+type T1 = NonNullable<undefined>;
+type T2 = null & {};
+type T3 = undefined & {};
+type T4 = null & undefined;
+type T6 = null & { a: string } & {};
+
+// Repro from #50519
+
+type NonNullableNew<T> = T & {};
+type NonNullableOld<T> = T extends null | undefined ? never : T;
+
+type TestNew = NonNullableNew<null>;
+type TestOld = NonNullableOld<null>;