--noUncheckedIndexedAccess

src/compiler/commandLineParser.ts

@@ -624,6 +624,14 @@ namespace ts {
             category: Diagnostics.Additional_Checks,
             description: Diagnostics.Report_errors_for_fallthrough_cases_in_switch_statement
         },
+        {
+            name: "noUncheckedIndexedAccess",
+            type: "boolean",
+            affectsSemanticDiagnostics: true,
+            showInSimplifiedHelpView: false,
+            category: Diagnostics.Additional_Checks,
+            description: Diagnostics.Include_undefined_in_index_signature_results
+        },
 
         // Module Resolution
         {

src/compiler/diagnosticMessages.json

@@ -4721,6 +4721,11 @@
         "code": 6504
     },
 
+    "Include 'undefined' in index signature results": {
+        "category": "Message",
+        "code": 6800
+    },
+
     "Variable '{0}' implicitly has an '{1}' type.": {
         "category": "Error",
         "code": 7005

src/compiler/types.ts

@@ -5783,6 +5783,7 @@ namespace ts {
         assumeChangesOnlyAffectDirectDependencies?: boolean;
         noLib?: boolean;
         noResolve?: boolean;
+        noUncheckedIndexSignatures?: boolean;
         out?: string;
         outDir?: string;
         outFile?: string;

tests/baselines/reference/api/tsserverlibrary.d.ts

@@ -2841,6 +2841,7 @@ declare namespace ts {
         assumeChangesOnlyAffectDirectDependencies?: boolean;
         noLib?: boolean;
         noResolve?: boolean;
+        noUncheckedIndexSignatures?: boolean;
         out?: string;
         outDir?: string;
         outFile?: string;

tests/baselines/reference/api/typescript.d.ts

@@ -2841,6 +2841,7 @@ declare namespace ts {
         assumeChangesOnlyAffectDirectDependencies?: boolean;
         noLib?: boolean;
         noResolve?: boolean;
+        noUncheckedIndexSignatures?: boolean;
         out?: string;
         outDir?: string;
         outFile?: string;

tests/baselines/reference/noUncheckedIndexedAccess.errors.txt

@@ -0,0 +1,109 @@
+tests/cases/conformance/pedantic/noUncheckedIndexedAccess.ts(3,32): error TS2344: Type 'boolean | undefined' does not satisfy the constraint 'boolean'.
+  Type 'undefined' is not assignable to type 'boolean'.
+tests/cases/conformance/pedantic/noUncheckedIndexedAccess.ts(38,1): error TS2322: Type 'undefined' is not assignable to type 'boolean'.
+tests/cases/conformance/pedantic/noUncheckedIndexedAccess.ts(39,1): error TS2322: Type 'undefined' is not assignable to type 'boolean'.
+tests/cases/conformance/pedantic/noUncheckedIndexedAccess.ts(40,1): error TS2322: Type 'undefined' is not assignable to type 'boolean'.
+tests/cases/conformance/pedantic/noUncheckedIndexedAccess.ts(41,1): error TS2322: Type 'undefined' is not assignable to type 'boolean'.
+tests/cases/conformance/pedantic/noUncheckedIndexedAccess.ts(85,1): error TS2322: Type 'undefined' is not assignable to type 'string'.
+
+
+==== tests/cases/conformance/pedantic/noUncheckedIndexedAccess.ts (6 errors) ====
+    type CheckBooleanOnly<T extends boolean> = any;
+    // Validate CheckBooleanOnly works - should error
+    type T_ERR1 = CheckBooleanOnly<boolean | undefined>;
+                                   ~~~~~~~~~~~~~~~~~~~
+!!! error TS2344: Type 'boolean | undefined' does not satisfy the constraint 'boolean'.
+!!! error TS2344:   Type 'undefined' is not assignable to type 'boolean'.
+
+    enum NumericEnum1 { A, B, C }
+    enum NumericEnum2 { A = 0, B = 1 , C = 2 }
+    enum StringEnum1 { A = "Alpha", B = "Beta" }
+
+    declare const strMap: { [s: string]: boolean };
+
+    // All of these should be errors
+    const e1: boolean = strMap["foo"];
+    const e2: boolean = strMap.bar;
+    const e3: boolean = strMap[0];
+    const e4: boolean = strMap[0 as string | number];
+    const e5: boolean = strMap[0 as string | 0 | 1];
+    const e6: boolean = strMap[0 as 0 | 1];
+    const e7: boolean = strMap["foo" as "foo" | "baz"];
+    const e8: boolean = strMap[NumericEnum1.A];
+    const e9: boolean = strMap[NumericEnum2.A];
+    const e10: boolean = strMap[StringEnum1.A];
+    const e11: boolean = strMap[StringEnum1.A as StringEnum1];
+    const e12: boolean = strMap[NumericEnum1.A as NumericEnum1];
+    const e13: boolean = strMap[NumericEnum2.A as NumericEnum2];
+    const e14: boolean = strMap[null as any];
+
+    // Should be OK
+    const ok1: boolean | undefined = strMap["foo"];
+    const ok2: boolean | undefined = strMap.bar;
+
+    type T_OK1 = CheckBooleanOnly<(typeof strMap)[string]>;
+    type T_OK2 = CheckBooleanOnly<(typeof strMap)["foo"]>;
+    type T_OK3 = CheckBooleanOnly<(typeof strMap)["bar" | "baz"]>;
+    type T_OK4 = CheckBooleanOnly<(typeof strMap)[number]>;
+    type T_OK5 = CheckBooleanOnly<(typeof strMap)[any]>;
+
+    // Writes don't allow 'undefined'; all should be errors
+    strMap["baz"] = undefined;
+    ~~~~~~~~~~~~~
+!!! error TS2322: Type 'undefined' is not assignable to type 'boolean'.
+    strMap.qua = undefined;
+    ~~~~~~~~~~
+!!! error TS2322: Type 'undefined' is not assignable to type 'boolean'.
+    strMap[0] = undefined;
+    ~~~~~~~~~
+!!! error TS2322: Type 'undefined' is not assignable to type 'boolean'.
+    strMap[null as any] = undefined;
+    ~~~~~~~~~~~~~~~~~~~
+!!! error TS2322: Type 'undefined' is not assignable to type 'boolean'.
+
+    // Numeric lookups are unaffected
+    declare const numMap: { [s: number]: boolean };
+    // All of these should be ok
+    const num_ok1: boolean = numMap[0];
+    const num_ok2: boolean = numMap[0 as number];
+    const num_ok3: boolean = numMap[0 as 0 | 1];
+    const num_ok4: boolean = numMap[NumericEnum1.A];
+    const num_ok5: boolean = numMap[NumericEnum2.A];
+
+    // Generics
+    function generic1<T extends { [s: string]: boolean }>(arg: T): boolean {
+        // Should error
+        return arg["blah"];
+    }
+    function generic2<T extends { [s: string]: boolean }>(arg: T): boolean {
+        // Should OK
+        return arg["blah"]!;
+    }
+    function generic3<T extends string>(arg: T): boolean {
+        // Should error
+        return strMap[arg];
+    }
+
+    // Element access into known properties is ok
+    declare const obj1: { x: string, y: number, [key: string]: string | number };
+    obj1["x"];
+    const y = "y";
+    obj1[y];
+    let yy = "y";
+    obj1[yy];
+    let z = "z";
+    obj1[z];
+
+    // Distributivity cases
+    declare const strMapUnion: { [s: string]: boolean } | { [s: string]: number };
+    // Should error
+    const f1: boolean | number = strMapUnion["foo"];
+
+    // Symbol index signatures
+    declare const s: unique symbol;
+    declare const symbolMap: { [s]: string };
+    const e15: string = symbolMap[s]; // Should OK
+    symbolMap[s] = undefined; // Should error
+    ~~~~~~~~~~~~
+!!! error TS2322: Type 'undefined' is not assignable to type 'string'.
+

tests/baselines/reference/noUncheckedIndexedAccess.js

@@ -0,0 +1,160 @@
+//// [noUncheckedIndexedAccess.ts]
+type CheckBooleanOnly<T extends boolean> = any;
+// Validate CheckBooleanOnly works - should error
+type T_ERR1 = CheckBooleanOnly<boolean | undefined>;
+
+enum NumericEnum1 { A, B, C }
+enum NumericEnum2 { A = 0, B = 1 , C = 2 }
+enum StringEnum1 { A = "Alpha", B = "Beta" }
+
+declare const strMap: { [s: string]: boolean };
+
+// All of these should be errors
+const e1: boolean = strMap["foo"];
+const e2: boolean = strMap.bar;
+const e3: boolean = strMap[0];
+const e4: boolean = strMap[0 as string | number];
+const e5: boolean = strMap[0 as string | 0 | 1];
+const e6: boolean = strMap[0 as 0 | 1];
+const e7: boolean = strMap["foo" as "foo" | "baz"];
+const e8: boolean = strMap[NumericEnum1.A];
+const e9: boolean = strMap[NumericEnum2.A];
+const e10: boolean = strMap[StringEnum1.A];
+const e11: boolean = strMap[StringEnum1.A as StringEnum1];
+const e12: boolean = strMap[NumericEnum1.A as NumericEnum1];
+const e13: boolean = strMap[NumericEnum2.A as NumericEnum2];
+const e14: boolean = strMap[null as any];
+
+// Should be OK
+const ok1: boolean | undefined = strMap["foo"];
+const ok2: boolean | undefined = strMap.bar;
+
+type T_OK1 = CheckBooleanOnly<(typeof strMap)[string]>;
+type T_OK2 = CheckBooleanOnly<(typeof strMap)["foo"]>;
+type T_OK3 = CheckBooleanOnly<(typeof strMap)["bar" | "baz"]>;
+type T_OK4 = CheckBooleanOnly<(typeof strMap)[number]>;
+type T_OK5 = CheckBooleanOnly<(typeof strMap)[any]>;
+
+// Writes don't allow 'undefined'; all should be errors
+strMap["baz"] = undefined;
+strMap.qua = undefined;
+strMap[0] = undefined;
+strMap[null as any] = undefined;
+
+// Numeric lookups are unaffected
+declare const numMap: { [s: number]: boolean };
+// All of these should be ok
+const num_ok1: boolean = numMap[0];
+const num_ok2: boolean = numMap[0 as number];
+const num_ok3: boolean = numMap[0 as 0 | 1];
+const num_ok4: boolean = numMap[NumericEnum1.A];
+const num_ok5: boolean = numMap[NumericEnum2.A];
+
+// Generics
+function generic1<T extends { [s: string]: boolean }>(arg: T): boolean {
+    // Should error
+    return arg["blah"];
+}
+function generic2<T extends { [s: string]: boolean }>(arg: T): boolean {
+    // Should OK
+    return arg["blah"]!;
+}
+function generic3<T extends string>(arg: T): boolean {
+    // Should error
+    return strMap[arg];
+}
+
+// Element access into known properties is ok
+declare const obj1: { x: string, y: number, [key: string]: string | number };
+obj1["x"];
+const y = "y";
+obj1[y];
+let yy = "y";
+obj1[yy];
+let z = "z";
+obj1[z];
+
+// Distributivity cases
+declare const strMapUnion: { [s: string]: boolean } | { [s: string]: number };
+// Should error
+const f1: boolean | number = strMapUnion["foo"];
+
+// Symbol index signatures
+declare const s: unique symbol;
+declare const symbolMap: { [s]: string };
+const e15: string = symbolMap[s]; // Should OK
+symbolMap[s] = undefined; // Should error
+
+
+//// [noUncheckedIndexedAccess.js]
+"use strict";
+var NumericEnum1;
+(function (NumericEnum1) {
+    NumericEnum1[NumericEnum1["A"] = 0] = "A";
+    NumericEnum1[NumericEnum1["B"] = 1] = "B";
+    NumericEnum1[NumericEnum1["C"] = 2] = "C";
+})(NumericEnum1 || (NumericEnum1 = {}));
+var NumericEnum2;
+(function (NumericEnum2) {
+    NumericEnum2[NumericEnum2["A"] = 0] = "A";
+    NumericEnum2[NumericEnum2["B"] = 1] = "B";
+    NumericEnum2[NumericEnum2["C"] = 2] = "C";
+})(NumericEnum2 || (NumericEnum2 = {}));
+var StringEnum1;
+(function (StringEnum1) {
+    StringEnum1["A"] = "Alpha";
+    StringEnum1["B"] = "Beta";
+})(StringEnum1 || (StringEnum1 = {}));
+// All of these should be errors
+var e1 = strMap["foo"];
+var e2 = strMap.bar;
+var e3 = strMap[0];
+var e4 = strMap[0];
+var e5 = strMap[0];
+var e6 = strMap[0];
+var e7 = strMap["foo"];
+var e8 = strMap[NumericEnum1.A];
+var e9 = strMap[NumericEnum2.A];
+var e10 = strMap[StringEnum1.A];
+var e11 = strMap[StringEnum1.A];
+var e12 = strMap[NumericEnum1.A];
+var e13 = strMap[NumericEnum2.A];
+var e14 = strMap[null];
+// Should be OK
+var ok1 = strMap["foo"];
+var ok2 = strMap.bar;
+// Writes don't allow 'undefined'; all should be errors
+strMap["baz"] = undefined;
+strMap.qua = undefined;
+strMap[0] = undefined;
+strMap[null] = undefined;
+// All of these should be ok
+var num_ok1 = numMap[0];
+var num_ok2 = numMap[0];
+var num_ok3 = numMap[0];
+var num_ok4 = numMap[NumericEnum1.A];
+var num_ok5 = numMap[NumericEnum2.A];
+// Generics
+function generic1(arg) {
+    // Should error
+    return arg["blah"];
+}
+function generic2(arg) {
+    // Should OK
+    return arg["blah"];
+}
+function generic3(arg) {
+    // Should error
+    return strMap[arg];
+}
+obj1["x"];
+var y = "y";
+obj1[y];
+var yy = "y";
+obj1[yy];
+var z = "z";
+obj1[z];
+// Should error
+var f1 = strMapUnion["foo"];
+var e15 = symbolMap[s]; // Should OK
+symbolMap[s] = undefined; // Should error