correctly recognize captured loop variables in destructuring assignment

src/compiler/checker.ts

@@ -7474,11 +7474,10 @@ namespace ts {
 
             // check if node is used as LHS in some assignment expression
             let isAssigned = false;
-            if (current.parent.kind === SyntaxKind.BinaryExpression) {
-                isAssigned = (<BinaryExpression>current.parent).left === current && isAssignmentOperator((<BinaryExpression>current.parent).operatorToken.kind);
+            if (isAssignmentTarget(current)) {
+                isAssigned = true;
             }
-
-            if ((current.parent.kind === SyntaxKind.PrefixUnaryExpression || current.parent.kind === SyntaxKind.PostfixUnaryExpression)) {
+            else if ((current.parent.kind === SyntaxKind.PrefixUnaryExpression || current.parent.kind === SyntaxKind.PostfixUnaryExpression)) {
                 const expr = <PrefixUnaryExpression | PostfixUnaryExpression>current.parent;
                 isAssigned = expr.operator === SyntaxKind.PlusPlusToken || expr.operator === SyntaxKind.MinusMinusToken;
             }

tests/baselines/reference/capturedLetConstInLoop12.js

@@ -0,0 +1,42 @@
+//// [capturedLetConstInLoop12.ts]
+(function() {
+    "use strict";
+
+    for (let i = 0; i < 4; i++) {
+        (() => [i] = [i + 1])();
+    }
+})();
+
+(function() {
+    "use strict";
+
+    for (let i = 0; i < 4; i++) {
+        (() => ({a:i} = {a:i + 1}))();
+    }
+})();
+
+//// [capturedLetConstInLoop12.js]
+(function () {
+    "use strict";
+    var _loop_1 = function(i) {
+        (function () { return (_a = [i + 1], i = _a[0], _a); var _a; })();
+        out_i_1 = i;
+    };
+    var out_i_1;
+    for (var i = 0; i < 4; i++) {
+        _loop_1(i);
+        i = out_i_1;
+    }
+})();
+(function () {
+    "use strict";
+    var _loop_2 = function(i) {
+        (function () { return (_a = { a: i + 1 }, i = _a.a, _a); var _a; })();
+        out_i_2 = i;
+    };
+    var out_i_2;
+    for (var i = 0; i < 4; i++) {
+        _loop_2(i);
+        i = out_i_2;
+    }
+})();

tests/baselines/reference/capturedLetConstInLoop12.symbols

@@ -0,0 +1,30 @@
+=== tests/cases/compiler/capturedLetConstInLoop12.ts ===
+(function() {
+    "use strict";
+
+    for (let i = 0; i < 4; i++) {
+>i : Symbol(i, Decl(capturedLetConstInLoop12.ts, 3, 12))
+>i : Symbol(i, Decl(capturedLetConstInLoop12.ts, 3, 12))
+>i : Symbol(i, Decl(capturedLetConstInLoop12.ts, 3, 12))
+
+        (() => [i] = [i + 1])();
+>i : Symbol(i, Decl(capturedLetConstInLoop12.ts, 3, 12))
+>i : Symbol(i, Decl(capturedLetConstInLoop12.ts, 3, 12))
+    }
+})();
+
+(function() {
+    "use strict";
+
+    for (let i = 0; i < 4; i++) {
+>i : Symbol(i, Decl(capturedLetConstInLoop12.ts, 11, 12))
+>i : Symbol(i, Decl(capturedLetConstInLoop12.ts, 11, 12))
+>i : Symbol(i, Decl(capturedLetConstInLoop12.ts, 11, 12))
+
+        (() => ({a:i} = {a:i + 1}))();
+>a : Symbol(a, Decl(capturedLetConstInLoop12.ts, 12, 17))
+>i : Symbol(i, Decl(capturedLetConstInLoop12.ts, 11, 12))
+>a : Symbol(a, Decl(capturedLetConstInLoop12.ts, 12, 25))
+>i : Symbol(i, Decl(capturedLetConstInLoop12.ts, 11, 12))
+    }
+})();

tests/baselines/reference/capturedLetConstInLoop12.types

@@ -0,0 +1,65 @@
+=== tests/cases/compiler/capturedLetConstInLoop12.ts ===
+(function() {
+>(function() {    "use strict";    for (let i = 0; i < 4; i++) {        (() => [i] = [i + 1])();    }})() : void
+>(function() {    "use strict";    for (let i = 0; i < 4; i++) {        (() => [i] = [i + 1])();    }}) : () => void
+>function() {    "use strict";    for (let i = 0; i < 4; i++) {        (() => [i] = [i + 1])();    }} : () => void
+
+    "use strict";
+>"use strict" : string
+
+    for (let i = 0; i < 4; i++) {
+>i : number
+>0 : number
+>i < 4 : boolean
+>i : number
+>4 : number
+>i++ : number
+>i : number
+
+        (() => [i] = [i + 1])();
+>(() => [i] = [i + 1])() : [number]
+>(() => [i] = [i + 1]) : () => [number]
+>() => [i] = [i + 1] : () => [number]
+>[i] = [i + 1] : [number]
+>[i] : [number]
+>i : number
+>[i + 1] : [number]
+>i + 1 : number
+>i : number
+>1 : number
+    }
+})();
+
+(function() {
+>(function() {    "use strict";    for (let i = 0; i < 4; i++) {        (() => ({a:i} = {a:i + 1}))();    }})() : void
+>(function() {    "use strict";    for (let i = 0; i < 4; i++) {        (() => ({a:i} = {a:i + 1}))();    }}) : () => void
+>function() {    "use strict";    for (let i = 0; i < 4; i++) {        (() => ({a:i} = {a:i + 1}))();    }} : () => void
+
+    "use strict";
+>"use strict" : string
+
+    for (let i = 0; i < 4; i++) {
+>i : number
+>0 : number
+>i < 4 : boolean
+>i : number
+>4 : number
+>i++ : number
+>i : number
+
+        (() => ({a:i} = {a:i + 1}))();
+>(() => ({a:i} = {a:i + 1}))() : { a: number; }
+>(() => ({a:i} = {a:i + 1})) : () => { a: number; }
+>() => ({a:i} = {a:i + 1}) : () => { a: number; }
+>({a:i} = {a:i + 1}) : { a: number; }
+>{a:i} = {a:i + 1} : { a: number; }
+>{a:i} : { a: number; }
+>a : number
+>i : number
+>{a:i + 1} : { a: number; }
+>a : number
+>i + 1 : number
+>i : number
+>1 : number
+    }
+})();

tests/cases/compiler/capturedLetConstInLoop12.ts

@@ -0,0 +1,15 @@
+(function() {
+    "use strict";
+
+    for (let i = 0; i < 4; i++) {
+        (() => [i] = [i + 1])();
+    }
+})();
+
+(function() {
+    "use strict";
+
+    for (let i = 0; i < 4; i++) {
+        (() => ({a:i} = {a:i + 1}))();
+    }
+})();