Cherry-pick PR #50704 into release-4.8 (#50741)

Component commits:
014fc0e Preserve special intersections in mapped types

5907d7c Add regression test

Co-authored-by: Anders Hejlsberg <andersh@microsoft.com>

src/compiler/checker.ts

@@ -11765,6 +11765,12 @@ namespace ts {
                 return mapType(type as UnionType, getLowerBoundOfKeyType);
             }
             if (type.flags & TypeFlags.Intersection) {
+                // Similarly to getTypeFromIntersectionTypeNode, we preserve the special string & {}, number & {},
+                // and bigint & {} intersections that are used to prevent subtype reduction in union types.
+                const types = (type as IntersectionType).types;
+                if (types.length === 2 && !!(types[0].flags & (TypeFlags.String | TypeFlags.Number | TypeFlags.BigInt)) && types[1] === emptyTypeLiteralType) {
+                    return type;
+                }
                 return getIntersectionType(sameMap((type as UnionType).types, getLowerBoundOfKeyType));
             }
             return type;

tests/baselines/reference/specialIntersectionsInMappedTypes.errors.txt

@@ -0,0 +1,21 @@
+tests/cases/compiler/specialIntersectionsInMappedTypes.ts(14,1): error TS2532: Object is possibly 'undefined'.
+
+
+==== tests/cases/compiler/specialIntersectionsInMappedTypes.ts (1 errors) ====
+    // Repro from #50683
+
+    type Alignment = (string & {}) | "left" | "center" | "right";
+    type Alignments = Record<Alignment, string>;
+
+    const a: Alignments = {
+        left: "align-left",
+        center: "align-center",
+        right: "align-right",
+        other: "align-other",
+    };
+
+    a.left.length;
+    a.other.length;  // Error expected here
+    ~~~~~~~
+!!! error TS2532: Object is possibly 'undefined'.
+

tests/baselines/reference/specialIntersectionsInMappedTypes.js

@@ -0,0 +1,28 @@
+//// [specialIntersectionsInMappedTypes.ts]
+// Repro from #50683
+
+type Alignment = (string & {}) | "left" | "center" | "right";
+type Alignments = Record<Alignment, string>;
+
+const a: Alignments = {
+    left: "align-left",
+    center: "align-center",
+    right: "align-right",
+    other: "align-other",
+};
+
+a.left.length;
+a.other.length;  // Error expected here
+
+
+//// [specialIntersectionsInMappedTypes.js]
+"use strict";
+// Repro from #50683
+var a = {
+    left: "align-left",
+    center: "align-center",
+    right: "align-right",
+    other: "align-other"
+};
+a.left.length;
+a.other.length; // Error expected here

tests/baselines/reference/specialIntersectionsInMappedTypes.symbols

@@ -0,0 +1,41 @@
+=== tests/cases/compiler/specialIntersectionsInMappedTypes.ts ===
+// Repro from #50683
+
+type Alignment = (string & {}) | "left" | "center" | "right";
+>Alignment : Symbol(Alignment, Decl(specialIntersectionsInMappedTypes.ts, 0, 0))
+
+type Alignments = Record<Alignment, string>;
+>Alignments : Symbol(Alignments, Decl(specialIntersectionsInMappedTypes.ts, 2, 61))
+>Record : Symbol(Record, Decl(lib.es5.d.ts, --, --))
+>Alignment : Symbol(Alignment, Decl(specialIntersectionsInMappedTypes.ts, 0, 0))
+
+const a: Alignments = {
+>a : Symbol(a, Decl(specialIntersectionsInMappedTypes.ts, 5, 5))
+>Alignments : Symbol(Alignments, Decl(specialIntersectionsInMappedTypes.ts, 2, 61))
+
+    left: "align-left",
+>left : Symbol(left, Decl(specialIntersectionsInMappedTypes.ts, 5, 23))
+
+    center: "align-center",
+>center : Symbol(center, Decl(specialIntersectionsInMappedTypes.ts, 6, 23))
+
+    right: "align-right",
+>right : Symbol(right, Decl(specialIntersectionsInMappedTypes.ts, 7, 27))
+
+    other: "align-other",
+>other : Symbol(other, Decl(specialIntersectionsInMappedTypes.ts, 8, 25))
+
+};
+
+a.left.length;
+>a.left.length : Symbol(String.length, Decl(lib.es5.d.ts, --, --))
+>a.left : Symbol(left)
+>a : Symbol(a, Decl(specialIntersectionsInMappedTypes.ts, 5, 5))
+>left : Symbol(left)
+>length : Symbol(String.length, Decl(lib.es5.d.ts, --, --))
+
+a.other.length;  // Error expected here
+>a.other.length : Symbol(String.length, Decl(lib.es5.d.ts, --, --))
+>a : Symbol(a, Decl(specialIntersectionsInMappedTypes.ts, 5, 5))
+>length : Symbol(String.length, Decl(lib.es5.d.ts, --, --))
+

tests/baselines/reference/specialIntersectionsInMappedTypes.types

@@ -0,0 +1,45 @@
+=== tests/cases/compiler/specialIntersectionsInMappedTypes.ts ===
+// Repro from #50683
+
+type Alignment = (string & {}) | "left" | "center" | "right";
+>Alignment : (string & {}) | "left" | "center" | "right"
+
+type Alignments = Record<Alignment, string>;
+>Alignments : { [x: string & {}]: string; left: string; center: string; right: string; }
+
+const a: Alignments = {
+>a : Alignments
+>{    left: "align-left",    center: "align-center",    right: "align-right",    other: "align-other",} : { left: string; center: string; right: string; other: string; }
+
+    left: "align-left",
+>left : string
+>"align-left" : "align-left"
+
+    center: "align-center",
+>center : string
+>"align-center" : "align-center"
+
+    right: "align-right",
+>right : string
+>"align-right" : "align-right"
+
+    other: "align-other",
+>other : string
+>"align-other" : "align-other"
+
+};
+
+a.left.length;
+>a.left.length : number
+>a.left : string
+>a : Alignments
+>left : string
+>length : number
+
+a.other.length;  // Error expected here
+>a.other.length : number
+>a.other : string | undefined
+>a : Alignments
+>other : string | undefined
+>length : number
+

tests/cases/compiler/specialIntersectionsInMappedTypes.ts

@@ -0,0 +1,17 @@
+// @strict: true
+// @noUncheckedIndexedAccess: true
+
+// Repro from #50683
+
+type Alignment = (string & {}) | "left" | "center" | "right";
+type Alignments = Record<Alignment, string>;
+
+const a: Alignments = {
+    left: "align-left",
+    center: "align-center",
+    right: "align-right",
+    other: "align-other",
+};
+
+a.left.length;
+a.other.length;  // Error expected here