Allow string enum in element access

src/compiler/checker.ts

@@ -7693,8 +7693,10 @@ namespace ts {
                 return type;
             }
             // In the following we resolve T[K] to the type of the property in T selected by K.
+            // We treat boolean as different from other unions to improve errors;
+            // skipping straight to getPropertyTypeForIndexType gives errors with 'boolean' instead of 'true'.
             const apparentObjectType = getApparentType(objectType);
-            if (indexType.flags & TypeFlags.Union && !(indexType.flags & TypeFlags.Primitive)) {
+            if (indexType.flags & TypeFlags.Union && !(indexType.flags & TypeFlags.Boolean)) {
                 const propTypes: Type[] = [];
                 for (const t of (<UnionType>indexType).types) {
                     const propType = getPropertyTypeForIndexType(apparentObjectType, t, accessNode, /*cacheSymbol*/ false);

tests/baselines/reference/stringEnumInElementAccess01.js

@@ -0,0 +1,26 @@
+//// [stringEnumInElementAccess01.ts]
+enum E {
+    A = "a",
+    B = "b",
+    C = "c",
+}
+
+interface Item {
+    a: string;
+    b: number;
+    c: boolean;
+}
+
+declare const item: Item;
+declare const e: E;
+const snb: string | number | boolean = item[e];
+
+
+//// [stringEnumInElementAccess01.js]
+var E;
+(function (E) {
+    E["A"] = "a";
+    E["B"] = "b";
+    E["C"] = "c";
+})(E || (E = {}));
+var snb = item[e];

tests/baselines/reference/stringEnumInElementAccess01.symbols

@@ -0,0 +1,40 @@
+=== tests/cases/conformance/expressions/elementAccess/stringEnumInElementAccess01.ts ===
+enum E {
+>E : Symbol(E, Decl(stringEnumInElementAccess01.ts, 0, 0))
+
+    A = "a",
+>A : Symbol(E.A, Decl(stringEnumInElementAccess01.ts, 0, 8))
+
+    B = "b",
+>B : Symbol(E.B, Decl(stringEnumInElementAccess01.ts, 1, 12))
+
+    C = "c",
+>C : Symbol(E.C, Decl(stringEnumInElementAccess01.ts, 2, 12))
+}
+
+interface Item {
+>Item : Symbol(Item, Decl(stringEnumInElementAccess01.ts, 4, 1))
+
+    a: string;
+>a : Symbol(Item.a, Decl(stringEnumInElementAccess01.ts, 6, 16))
+
+    b: number;
+>b : Symbol(Item.b, Decl(stringEnumInElementAccess01.ts, 7, 14))
+
+    c: boolean;
+>c : Symbol(Item.c, Decl(stringEnumInElementAccess01.ts, 8, 14))
+}
+
+declare const item: Item;
+>item : Symbol(item, Decl(stringEnumInElementAccess01.ts, 12, 13))
+>Item : Symbol(Item, Decl(stringEnumInElementAccess01.ts, 4, 1))
+
+declare const e: E;
+>e : Symbol(e, Decl(stringEnumInElementAccess01.ts, 13, 13))
+>E : Symbol(E, Decl(stringEnumInElementAccess01.ts, 0, 0))
+
+const snb: string | number | boolean = item[e];
+>snb : Symbol(snb, Decl(stringEnumInElementAccess01.ts, 14, 5))
+>item : Symbol(item, Decl(stringEnumInElementAccess01.ts, 12, 13))
+>e : Symbol(e, Decl(stringEnumInElementAccess01.ts, 13, 13))
+

tests/baselines/reference/stringEnumInElementAccess01.types

@@ -0,0 +1,44 @@
+=== tests/cases/conformance/expressions/elementAccess/stringEnumInElementAccess01.ts ===
+enum E {
+>E : E
+
+    A = "a",
+>A : E.A
+>"a" : "a"
+
+    B = "b",
+>B : E.B
+>"b" : "b"
+
+    C = "c",
+>C : E.C
+>"c" : "c"
+}
+
+interface Item {
+>Item : Item
+
+    a: string;
+>a : string
+
+    b: number;
+>b : number
+
+    c: boolean;
+>c : boolean
+}
+
+declare const item: Item;
+>item : Item
+>Item : Item
+
+declare const e: E;
+>e : E
+>E : E
+
+const snb: string | number | boolean = item[e];
+>snb : string | number | boolean
+>item[e] : string | number | boolean
+>item : Item
+>e : E
+

tests/cases/conformance/expressions/elementAccess/stringEnumInElementAccess01.ts

@@ -0,0 +1,16 @@
+// @noImplicitAny: true
+enum E {
+    A = "a",
+    B = "b",
+    C = "c",
+}
+
+interface Item {
+    a: string;
+    b: number;
+    c: boolean;
+}
+
+declare const item: Item;
+declare const e: E;
+const snb: string | number | boolean = item[e];