Element access assignment uses declared type, not narrowed type

src/compiler/checker.ts

@@ -9305,7 +9305,9 @@ namespace ts {
                         }
                     }
                     const propType = getTypeOfSymbol(prop);
-                    return accessExpression ? getFlowTypeOfReference(accessExpression, propType) : propType;
+                    return accessExpression && getAssignmentTargetKind(accessExpression) !== AssignmentKind.Definite ?
+                        getFlowTypeOfReference(accessExpression, propType) :
+                        propType;
                 }
                 if (isTupleType(objectType)) {
                     const restType = getRestTypeOfTupleType(objectType);

tests/baselines/reference/checkJsxChildrenCanBeTupleType.js

@@ -30,7 +30,7 @@ var __extends = (this && this.__extends) || (function () {
             ({ __proto__: [] } instanceof Array && function (d, b) { d.__proto__ = b; }) ||
             function (d, b) { for (var p in b) if (b.hasOwnProperty(p)) d[p] = b[p]; };
         return extendStatics(d, b);
-    }
+    };
     return function (d, b) {
         extendStatics(d, b);
         function __() { this.constructor = d; }

tests/baselines/reference/controlFlowElementAccess.js

@@ -0,0 +1,21 @@
+//// [controlFlowElementAccess.ts]
+let x: { o: boolean } = { o: false }
+if (x['o'] === false) {
+    x['o'] = true
+}
+
+const y: [number, number] = [0, 0];
+if (y[0] === 0) {
+    y[0] = -1;
+}
+
+
+//// [controlFlowElementAccess.js]
+var x = { o: false };
+if (x['o'] === false) {
+    x['o'] = true;
+}
+var y = [0, 0];
+if (y[0] === 0) {
+    y[0] = -1;
+}

tests/baselines/reference/controlFlowElementAccess.symbols

@@ -0,0 +1,27 @@
+=== tests/cases/conformance/controlFlow/controlFlowElementAccess.ts ===
+let x: { o: boolean } = { o: false }
+>x : Symbol(x, Decl(controlFlowElementAccess.ts, 0, 3))
+>o : Symbol(o, Decl(controlFlowElementAccess.ts, 0, 8))
+>o : Symbol(o, Decl(controlFlowElementAccess.ts, 0, 25))
+
+if (x['o'] === false) {
+>x : Symbol(x, Decl(controlFlowElementAccess.ts, 0, 3))
+>'o' : Symbol(o, Decl(controlFlowElementAccess.ts, 0, 8))
+
+    x['o'] = true
+>x : Symbol(x, Decl(controlFlowElementAccess.ts, 0, 3))
+>'o' : Symbol(o, Decl(controlFlowElementAccess.ts, 0, 8))
+}
+
+const y: [number, number] = [0, 0];
+>y : Symbol(y, Decl(controlFlowElementAccess.ts, 5, 5))
+
+if (y[0] === 0) {
+>y : Symbol(y, Decl(controlFlowElementAccess.ts, 5, 5))
+>0 : Symbol(0)
+
+    y[0] = -1;
+>y : Symbol(y, Decl(controlFlowElementAccess.ts, 5, 5))
+>0 : Symbol(0)
+}
+

tests/baselines/reference/controlFlowElementAccess.types

@@ -0,0 +1,45 @@
+=== tests/cases/conformance/controlFlow/controlFlowElementAccess.ts ===
+let x: { o: boolean } = { o: false }
+>x : { o: boolean; }
+>o : boolean
+>{ o: false } : { o: false; }
+>o : false
+>false : false
+
+if (x['o'] === false) {
+>x['o'] === false : boolean
+>x['o'] : boolean
+>x : { o: boolean; }
+>'o' : "o"
+>false : false
+
+    x['o'] = true
+>x['o'] = true : true
+>x['o'] : boolean
+>x : { o: boolean; }
+>'o' : "o"
+>true : true
+}
+
+const y: [number, number] = [0, 0];
+>y : [number, number]
+>[0, 0] : [number, number]
+>0 : 0
+>0 : 0
+
+if (y[0] === 0) {
+>y[0] === 0 : boolean
+>y[0] : number
+>y : [number, number]
+>0 : 0
+>0 : 0
+
+    y[0] = -1;
+>y[0] = -1 : -1
+>y[0] : number
+>y : [number, number]
+>0 : 0
+>-1 : -1
+>1 : 1
+}
+

tests/baselines/reference/unionTypeWithIndexSignature.errors.txt

@@ -1,13 +1,12 @@
 tests/cases/conformance/types/union/unionTypeWithIndexSignature.ts(11,3): error TS2339: Property 'bar' does not exist on type 'Missing'.
   Property 'bar' does not exist on type '{ [s: string]: string; }'.
 tests/cases/conformance/types/union/unionTypeWithIndexSignature.ts(14,4): error TS2540: Cannot assign to 'foo' because it is a constant or a read-only property.
-tests/cases/conformance/types/union/unionTypeWithIndexSignature.ts(18,1): error TS2322: Type '"ok"' is not assignable to type 'number'.
 tests/cases/conformance/types/union/unionTypeWithIndexSignature.ts(24,1): error TS7017: Element implicitly has an 'any' type because type 'Both' has no index signature.
 tests/cases/conformance/types/union/unionTypeWithIndexSignature.ts(25,1): error TS2322: Type '"not ok"' is not assignable to type 'number'.
 tests/cases/conformance/types/union/unionTypeWithIndexSignature.ts(26,1): error TS7017: Element implicitly has an 'any' type because type 'Both' has no index signature.
 
 
-==== tests/cases/conformance/types/union/unionTypeWithIndexSignature.ts (6 errors) ====
+==== tests/cases/conformance/types/union/unionTypeWithIndexSignature.ts (5 errors) ====
     type Two = { foo: { bar: true }, baz: true } | { [s: string]: string };
     declare var u: Two
     u.foo = 'bye'
@@ -31,8 +30,6 @@ tests/cases/conformance/types/union/unionTypeWithIndexSignature.ts(26,1): error
     declare var num: Num
     num[0] = 1
     num['0'] = 'ok'
-    ~~~~~~~~
-!!! error TS2322: Type '"ok"' is not assignable to type 'number'.
     const sym = Symbol()
     type Both = { s: number, '0': number, [sym]: boolean } | { [n: number]: number, [s: string]: string | number }
     declare var both: Both

tests/baselines/reference/unionTypeWithIndexSignature.types

@@ -96,7 +96,7 @@ num[0] = 1
 
 num['0'] = 'ok'
 >num['0'] = 'ok' : "ok"
->num['0'] : number
+>num['0'] : string | number
 >num : Num
 >'0' : "0"
 >'ok' : "ok"

tests/cases/conformance/controlFlow/controlFlowElementAccess.ts

@@ -0,0 +1,9 @@
+let x: { o: boolean } = { o: false }
+if (x['o'] === false) {
+    x['o'] = true
+}
+
+const y: [number, number] = [0, 0];
+if (y[0] === 0) {
+    y[0] = -1;
+}